Abstract: In this work, stationary shoulder was used in friction stir lap welding (FSLW) of dissimilar magnesium alloy to aluminum alloy. Effect of the welding speed on microstructure and mechanical properties of the lap joint were mainly studied. Results show that the stationary shoulder can enhance the material mixing between the two alloys. The stir zone (SZ) is characterized by a big onion ring after welding, in which plenty of intermetallic compounds (IMC) can be obtained. Excellent metallurgical bonding can be observed at the lap interface. Microhardness of the onion ring is higher than other regions of the joint due to the IMC. X-ray diffraction (XRD) pattern analysis shows that the main IMCs formed in the SZ are Al₃Mg₂ and Al₁₂Mg₁₇. Lap shear failure load of the joint firstly increases and then decreases with increasing the welding speed. The maximum load is obtained when using 40 mm/min.