Abstract: The main welding defects in the SiCp/6061Al composite resistance spot welding (RSW) joint and the generating causes were investigated. The results show the main welding defects in the composite RSW joint were hydrogen pore, hot crack and welding spatter. That the composites were not cleaned thoroughly and the long welding time(0.3 s) led to the forming of hydrogen pore. The hot cracks forming reason was the excessive welding current (16 kA) which led to the nonuniform temperature field when the nugget crystallized, and the increase of the welding stress. The SiC particles in the welding pool made the increase of the viscosity during the nugget crystallization. The local overheating of nugget zone resulted in the excessive current (16 kA) was the reason for welding spatter.