The eutectic Sn-0.7mass%Cu solder is one of the promising lead-free solder for wave-soldering applications. In this study the interfacial reaction and the joint strength between Sn-0.7Cu solder and Cu base metal was investigated. Sn-0.7Cu-xNi solders (x = 0, 0.05, 0.1, 0.2 mass%) was used to reveal the effect of Ni minor addition on the formation of intermetallic compounds(IMC). A spread test specimen was used to observe the interfacial reaction between solder and copper. The cross sections of the soldered specimens were observed with optical microscopy, scanning electron microscopy and electron probe X-ray microanalyzer to measure the IMC thickness and the dissolution thickness of Cu base metal and to investigate the microstructures of IMC. And then shear test was performed to measure the joint strength between Sn-Cu (-Ni) solders and Cu base metal. The experimental results showed that the dissolution thickness of Cu base metal was increased with the increase of Ni content in Sn-0.7Cu solder and then the IMC thickness between Sn-Cu (-Ni) solders and Cu base metal was minimum in Sn-Cu 0.05Ni solder just after soldering. The IMC grew with increasing of aging time strongly depended on the Ni concentration. When Ni concentration was 0.2%, the IMC growth rate was the largest and the thickness was about 6µm after aging at 423K for 504h. EPMA analysis confirmed that the IMC layer contained Ni and the composition of IMC can be expressed as (Cu(1-x), Nix)6Cu5.