The synthesis of semiconducting nanoplates (NPs) with defined crystal phase is of particular interest, especially their intriguing properties related to the size, shape, and crystal phase. Herein, a liquid-state transformation process from hexagonal-phase CuS NPs is employed to fabricate the cubic-phase Cu2S NPs. The CuS NPs were converted into Cu2S NPs but maintained the morphology. The Cu2S NPs exhibit better oxygen evolution reaction (OER) activity than CuS NPs. Furthermore, the OER activity of Cu2S NPs can be improved by the addition of a glycine (Gly) solution. The Cu2S NPs with Gly in a phosphate buffer solution exhibit excellent OER activity and durability, which approaches that of the best known commercial Ir/C (20%) nanocatalyst. In this Work, a good strategy for fabricating a noble-metal-free OER catalyst has been proposed, which could provide insight into developing new water oxidation catalysts with high activity.