Interfacial reaction between electroless plated Ni-P/Au UBM(Under Bump Metallization and eutectic Sn-58mass%Bi solder was studied by using AEM(Analytical Electron Microscopy). UBM is prepared by the electroless plating of Au (0.15μm ) / Ni-15at%P (7 μm ) on bare Cu substrate, and then it is reacted with Sn-58Bi solder at 220°C for 1 min. The chemical analysis using AEM provided us very consequential information about microstructure of the interface and phases formed. CBED(Convergent Beam Electron Diffraction) technique is used for phase identification of intermetallic compounds. In this study, the AEM results indicate that Ni3Sn is formed at the P-rich Ni layer/Ni3Sn4 interface by crystallographic analysis. The measured primitive cell volume(104.10 Å3)of this phase is close to the Ni3Sn(103.19 Å3) rather than Ni2SnP(235.24 Å3).