The following article describes results of investigations on influence of laser welding parameters on the weld shape, quality and mechanical properties of 2.5 mm thick butt joints of thermo-mechanically rolled, high yield strength steel for cold forming S420MC (according to EN 10149 - 3 and 060XLK according to ASTM) welded with high power diode laser HPDL ROFIN SINAR DL 020 with rectangular laser beam spot and 2.2 kW output power, and 808 nm wavelength. The investigations at the initial stage were focused on detailed analysis of influence of the basic laser welding parameters such as laser power and welding speed on the shape and quality of single bead produced during bead-on-plate welding. Then the optimal parameters were chosen for laser welding of 2.5 mm thick butt joints of the thermo-mechanically rolled, high yield strength steel sheets for cold forming S420MC. The test joints were prepared as single square groove and one-side laser welded without an additional material, at a flat position. Edges of steel sheets were melted in argon atmosphere by the laser beam focused on the top joint surface. The test welded joints were investigated by visual inspection, metallographic examinations, mechanical tests such as tensile tests and bending tests. It was found that the high power diode laser may be applied successfully for one-side welding of the S420MC steel butt joints. Additionally it was found that in the optimal range of laser welding parameters the high quality joint were produced.