In the doctoral thesis, the properties of welded seams created by welding several types of thermoplastic polymer materials using high-tech methods of welding clothing parts were investigated. The welding method includes, in addition to the welding technique, machine handling, and a detailed analysis of welding process parameters. The welding techniques that were used to make the welded seam samples that were examined in the doctoral thesis are the ultrasonic welding technique, high-frequency welding technique, thermal welding by conduction, thermal welding by convection and a combination of sewing and thermal welding by convection, joining adhesive tape by sewing seam. Welded seams joined by high-tech welding techniques can have, depending on the material sample and the way the welded seam is designed, the property of air tightness and/or water tightness. Such welded seams are used when joining parts of clothing for specific purposes, such as protective and intelligent clothing. For the welding of thermoplastic polymer materials, the welding parameters are defined, applied for welding samples of thermoplastic materials. Evaluation of the quality of welded seam based on the visual appearance of the welded seam was evaluated in two phases. In the first evaluation phase, those welded seam samples where the weld was achieved were given a positive grade, and all those welded seam samples that were weakly weld or during welding were damaged in the weld zone and/or the surrounding area were given a negative grade. In the second evaluation phase, the welded seam samples were assigned a grade from 1 (bad welded seam) to 5 (good welded seam), based on the criteria of damage (O) or wrinkling of material in the weld zone or next to the weld (N). Tests of the properties of the welded seams were carried out on the samples of weld that were evaluated with a good visual quality rating. The paper presents the results of properties measurement on KES-FB (Kawabata Evaluation System) measuring devices: tensile, shear, bending, and compression properties of welded seams. Based on the analysis of the measurement results of the examined welded seams properties, the values of which change depending on the applied welding process parameters, the relationship between the influence of the welding process parameters on the specified welded seam properties, the aesthetic appearance and the quality of the welded seam was determined. In addition to the listed properties of the welded seams, the quality of the weld is also affected by the strength of the welded seam, the values of which were tested and the relationship between the influence of the welding parameters on the strength of the welded seam was also analysed. By studying the relationship between the influence of the welding parameters on the values of the measured parameters of the compression properties of welded seams, it was observed that a very important factor that affects the values of all studied properties of the welded seams is the thickness of the extruded edges. Lower values of the thickness of extruded edges indicate that the welded seam has slightly extruded edges, and higher measured values indicate a higher intensity of extruded edges. The highly accentuated extruded edges are deformations that occur when a large amount of heat is introduced and/or initiated in the welding zone. The softened material is pushed out of the area affected by the heat, by the action of pressure force, into the edges of the welding seam. From the results of testing the strength of the welded seams and the influence of different welding parameters, it can be concluded that the strength of the welded seams is significantly influenced by the welding parameters as well as the number of the material layers and their position. The highest values of welded seam strength were measured for welded seams, welded at lower values of the welding speed (which corresponds to a longer time of energy action on the material) and higher values of the strength anode current/ultrasonic power. Welded seams that have higher values of the thickness of the extruded edges have a higher strength. In order to determine the amount of energy exerted on the material during welding, calculations were carried out based on the input welding parameters, energy density values per volume were calculated. By comparing the values, data was obtained on the amount of energy that needs to be used to achieve a quality welded seam for each of the material samples. The analysis determined that the welded seams where it was calculated that they initiate a higher energy density per volume have higher values of the welded seams strength and a higher values of the thickness of the extruded edge.