Testing of laser welded joints made in steel grades containing increased content of carbon revealed that the use of active shielding gas mixtures does not necessarily have harmful effect on mechanical properties of the welded joint. This thesis studies the effects of the type of active shielding gas mixtures based on argon with additions of oxygen and/or carbon dioxide upon the geometry and mechanical properties of the welded joint. In the introduction, based on literature data, the physical bases and the laser welding technology have been discussed. Factors affecting the welding process and the quality of the welded joint have been studied. The methods used in the quality control during laser welding have been mentioned. Also, the laser weldability of steels has been discussed. In the experimental part, the heat-treatable steels of grade 25 CrMo 4, 42 CrMo 4 and Ck 60 (DIN EN 10083-1) have been welded. The sheets of 2 mm thickness have been used. Joint configuration was of butt design, and welding was done by applying cw Nd:YAG laser unit rated at 2000 W of mean power, with laser beam guidance by means of optic fiber of 0.6 mm in diameter, applying focusing optics 120/120 mm that allows beam focusing to 0.6 mm spot. Shielding mixtures were supplied through 5 mm dia. coaxial nozzle. The relative movement of the welding head was performed by "IGM limat RT 280-6" robot. The rated power was set at 1800 W level, at power density of 6.4•105 W/cm2. Trial welding was carried out according to the experimental model with mixtures. The welded joints were inspected visually and tested by applying radiographic method. The geometric features of the joint were measured. Hardness, tensile strength, deformability, impact toughness and dynamic strength were tested as well. The obtained data were statistically processed, and mathematical modeling applying the method of response surfaces was carried out. The quality of the welded joints was assessed by applying the HRN EN ISO 13919-1 standard. The produced welds were of high quality, without occurrence of cracks. The analysis revealed a significant impact of the shielding gas mixtures upon the geometric shape of the joint, whereas the effect on the welding speed was not significant. Neither was the effect of shielding mixtures upon mechanical properties of the joint significant. The exception was recorded for dynamic strength, where significant improvement was achieved when the mixtures Ar/O2 and Ar/CO2/O2 enabling root shape with reduced notch effect were applied. daju oblik korijena zavara s manjim zareznim djelovanjem.