Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) were employed to study the microstructure and transformation of Cu-Ni-Si alloy. In Cu-1.0Ni-0.25Si-0.1Zn (wt.%) alloy, the precipitate responsible for the age-hardening effect was the δ-Ni 2 Si originating in nucleation process. On aging at the temperature below ~773 K in Cu-3.2Ni-0.75Si-0.3Zn (wt.%) alloy, there were three different transformation products: a modulated structure resulting from spinodal decomposition, a (Cu, Ni) 3 Si with DO 2 2 ordering structure nucleating from the modulated structure, and a δ-Ni 2 Si phase with disc-like structure appearing in (Ni, Si)-rich regions. On aging at the temperature above ~773 K, the transformation products were grain boundary and intragranular precipitations of δ-Ni 2 Si. The free energy versus composition curves were employed to further explain the experimental observations.