Abstract: C18000 copper alloy plate with thickness of 3.5 mm was subjected to friction stir welding experiment. Sound welded joints were obtained under the welding speed of 120 mm/min and the rotation speed of 1 200 r/min. The macro-morphology and micro-structure of the joint were observed under metallographic microscope, and the microstructure of the base metal and stirring zone were observed and analyzed by scanning electron microscopy and transmission electron microscopy. The results showed that the joint zone could be generally divided into the base metal (BM), the heat affected zone (HAZ), the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ). The grain in SZ was small and well-distributed, and the grain in TMAZ was elongated along the tangential direction of the boundary. It was found that the particles were Cr₃Si phase, and it was dissolved in the stirring zone. The Cr element phase and Ni₂Si phase dissolved in the microstructure of SZ, which resulted in a decrease in joint hardness and tensile strength. The average hardness of the SZ was 151. 4 HV; the tensile strength of the joint was 497 MPa, which was 72% of the BM. The conductivity of the joint dropped to 35% IACS.