The purpose of this paper is to study the effect of nickel plating thickness on fatigue failure of lead-free solder joints after thermal aging. Cyclic three-point bending tests were carried out and investigate the relationships between intermetallic compound(IMC) layer development and the fatigue life of solder joints. After the fatigue tests, crosssections of solder joint were observed by scanning electron microscope(SEM) and crack propagation paths were investigated. The results show that when the thickness of nickel plating layer is too thin, voids in solder were observed and the fatigue life of solder joints were short. On the other hands, when the thickness of nickel plating layer is too thick, thick IMC layers were formed between solder and plating layers and developed after thermal aging. The optimum thickness of nickel plating layer in this study was 0.08 or 0.12 pim. In these cases, the nickel layer diffused into the solder and was consumed completely to form IMC layer during reflow process. Then, the development of IMC layer was not occurred and good fatigue strengths were obtained.