In this study, the experimental apparatus with a friction stir welding dynamometer was employed to investigate the joint characteristics of Cu-Ni alloy plate in thickness of 3.6mm lap-welding to low carbon steel plate in thickness of 4 mm using cylinder type tool (without probe) under the welding parameters of rotating speeds (800~1400 rpm) and traveling speed of tool (10~80 mm/min). To prevent the joint interface from oxidizing during the welding process, the joint interfaces of Cu-Ni alloy and low carbon steel respectively were electroplated with Ni coating layer in different thicknesses before the welding. The effect of the thickness of Ni coating layer on shear strength of joint interface and the mechanism of welding are also investigated. Experimental results show that under the rotating speed of 1000 rpm and travelling speed of 10 mm/min, the shear strength for without Ni coating layer is measured about 100 MPa. On the other hand, the shear strength is increased to saturated value of 290 MPa with increasing the thickness of Ni coating layer. Especially, the shear strength of joint interface for the Cu-Ni alloy with 5£gm thickness of Ni coating layer lap-welding to low carbon steel with thickness of 20£gm thickness of Ni coating layer is about 2.9 times of that for without Ni coating layer. Moreover, the downward force (Fd) is decreased and the maximum interface temperature (Tmax) and shear strength (£n) are increased with increasing the rotating speed (N). The downward force is increased and the maximum interface temperature and shear strength are decreased with increasing the traveling speed (f). This complex relationship is discussed by the new parameter of Fd¡EN/f, the relationship among Fd¡EN/f, maximum interface temperature and shear strength shows that the maximum interface temperature is increased and shear strength is increased to saturated value of 290 MPa with increasing Fd¡EN/f. The phenomenon is explained that the diffusion bonding between the joint interface of two plates become more homogeneous.