Resistance spot welding (RSW) was used to weld Mg alloy AZ61A. To research weldability of this alloy many welding parameters, e.g. current, time, pressure, have been tried in wide range on the welding source DALEX PMS 11 – 4. Visual check was followed with metallography, tensile test etc. Some defects as solidification, liquation cracks, porosity were found. The creation of imperfections was found to be dependent on parameters, so finally optimized parameters were found. The best influence on weld integrity, i.e. weld w/o cracks, was to set cooling time directly after welding. For sheet metal of 1 mm optimum welding parameters were 8 kA, 100 ms, 9 kN and long cooling time. Another problem found, was limited lifetime of Cu electrodes, when in contact with Mg melt. This contamination damagedsurface quality of electrodes, further influencing weld quality. High enough electrode pressure can diminish this problem.