In the Sn/Co interfacial system, the formed CoSn3 grows fast with a linear growth rate. In this study, minor Ga addition in Sn solder was found to have strong effects on the Sn/Co interface, including IMC (intermetallic compound) growth inhibition and phase transition of formed IMCs. Liquid-state Sn-Ga/Co reactions were examined at 250oC with various Ga contents. With only 0.2wt%Ga addition, the CoSn3 growth was greatly suppressed by 95%, which is possibly because minor doping-Ga atoms in the CoSn3 phase hinder the fast diffusion of Sn atoms. With 0.5wt%Ga addition, only a thin CoGa layer of ~1 μm thick was stably present and it had no significant growth even after aging for 24 h. In solid-state reaction, the inhibition was more pronounced. Additionally, the reaction phase changed to the metastable CoSn4. With only 0.02wt%Ga addition, the CoSn4 growth at 160oC was strongly suppressed by ~95%.