Abstract: Objective The influence on microstructure and properties of stud welded joints in relation to oil coating amount on aluminum alloy surfaces was investigated. Methods Using AA5182 aluminum alloy that commonly used for automotive panels as the substrate, by means of macroscopic morphologies, grain microstructure, mechanical properties, and fracture analysis, changes of microstructure and mechanical properties of stud welding during the variation of surface oil coating amount from 0 to 1.00 mg/m² were studied. Results The results indicated that stud welded joints were divided into weld zones, fusion zones, and heat affected zones. Weld zones exhibited equiaxed grain microstructure, while fusion zones displayed columnar grain microstructure. The width of heat affected zones was relatively small, and there was no significant grain growth. As oil coating amount applied to the substrate increased, the number and size of pores in the stud welding pools increased dramatically. As oil coating amount increased to 0.50 mg/m², there existed more pore defects in the welding pools, and welding pools overflowed significantly, which caused the flange of stud melting completely. Welding pools produced obvious welding spatter, and the number and size of pores increased significantly. With the increase of oil coating amount on the substrate surface, tensile resistance of welded joints decreased from 3 815 N to 3 168 N with a decrease of 17.0%, and destructive torque decreased from 9.18 N·m to 6.94 N·m with a decrease of 24.4%. Conclusion Oil coating amount on the substrate surface had significant impact on porosity defects and mechanical properties in stud welding. To ensure joining quality of stud welding, oil coating amount on the aluminum alloy surface should be less than 0.50 mg/m².