A new series of magnesium alloys are being developed as due to good properties with any rare earth components. In order to investigate the effects of friction stir welding (FSW) on microstructures and thermo gravimetric analysis of ZE42 magnesium alloy, the hot-extruded alloy plates were butt welded by FSW at various welding speeds.The results showed that the alloy was jointed without defects. The alloy is characterized by mechanical properties, SEM, XRD, TGA and FTIR. With increasing welding speed, the maximum intensity in the NZ first decreased and then increased. In all cases the tensile properties of the alloy after FSW decreased due to the softened region at the heataffected zone (HAZ), FT-IR results helps in distinguishing strong absorption appear in the region of nugget zone it is easily recognized by bands.