Ultrasonic wave was applied to join cold rolled carbon steel to aluminum. The effect of ultrasonic wave was investigated by measuring the fracture strenth of the steel/aluminum joint, and observing the microstructure of the joint. The application of ultrasonic wave was effective to improve the joining strength of the steel/aluminum joint during botn brazing and bonding processes. The short time of applying the ultrasonic wave effectively improve to increase the joining strength of the steel/aluminum joint during brazing or bonding. The observation of the interface of tjhe stele/aluminum joint showed the difuusion of iron and aluminum at the interface between stele and aluminum. The application of yultrasonic wave acceleration of  the diffusion of elements at the interface. This accelerartion of diffusion of elements at the interface between steel and aluminum improve the joining strength of the joint.