The surface structures of the electrocatalyst have great influence on its catalytic activity. In this study, the surface structures of NiMoO4 particles were altered through a post-phosphorization treatment to enhance the electrocatalytic activity of NiMoO4 for oxygen evolution reaction (OER). After the post-phosphorization treatment, an amorphous layer containing a large number of electrocatalytic active sites such as the phosphate, NiP bonds as well as the oxygen vacancies was formed on the surface of crystalline NiMoO4 particles. As a result, the electrocatalytic activity of the phosphorized NiMoO4 for OER was significantly improved.