This thesis is consisted of a theoretical and an experimental part. Theoretical part of paper defined and described non - destructive testing methods for which was also individually listed their advantages and disadvantages. Other than that for each method is also listed characteristic risks and areas of application. Special attention was given to the detailed description of the ultrasonic method used in experimental part of work. In addition to the description of the NDT method, the paper also deals with the characteristic errors in welded joints. The groups of some relevant faults as well as the causes of their occurrence are described in more detail, such as cracks, porosities, overhangs, involved, welds and similar. In paper is also described their effects on solid material or welded joints. It was concluded that most defects, and especially cracks and porosities when the size of the defect exceeds the critical value, can have a significantly negative impact on the strength and functionality of the elements. In the experimental part was described in detail the procedure of ultrasonic testing of the welded joint of the 15 mm thick plates. The test was performed with pulse - echo technique while using a USM GO ultrasound device from General Electric and a 60˚ angle probe. After testing the obtained results were recorded and analyzed. It was concluded that on the face of weld and weld toe is detected discontinuity that indicate the presence of porosity in the welded.