Abstract: Using ABAQUS software and DFLUX subroutine interface, a numerical model of the heat source was established for the friction stir welding of AA2195 Al-Li alloy and AZ31B Mg alloy. The effect of welding parameters on the temperature distribution was studied and it was compared with the experimental results. The results show that the best weld is obtained at rotation speed of 2000 r/min and welding speed of 900 mm/min. In the transverse temperature distribution vertical to the weld, the peak temperature appears at the side of the AZ31B Mg alloy about 1 mm away from the joint line, while the highest temperature at AA2195 Al-Li alloy side is at the joint line of the weld center. The highest temperature along the weld depth has a tendency to shift towards the joint line. The peak temperature is inversely correlated with the welding speed and positively correlated with the rotation speed. The effect of the welding speed on the temperature is more significant than that of the rotation speed. The quality of the weld improves as the welding speed increases. The formation of adhesive and flash defects is described by the Arrhenius equation, which shows that the probability of adhesive and flash defects formation increases as the welding temperature increases.