The strengthening precipitates in a Cu-Ni-P alloy have been investigated using aberration-corrected transmission electron microscope. It is found that coherent plate-shaped particles with hexagonal Ni2P (h- Ni2P) structure are mainly responsible for the hardness of this alloy. The orientation relationship between h-Ni2P precipitates and the matrix should be revised as (111)Cu//(12¯11)h, [11¯0]Cu//[101¯0]h and [112¯]Cu//[12¯16¯]h, and the habit plane of them is parallel to the {111}Cu. Both the morphological and crystallographic characteristics of the h-Ni2P precipitates can be explained by classical theories of phase transformation.