Nickel phosphide nanostructures are synthesized by hydrothermal method. The effect of different nickel phosphide phases on structural, morphological, and optical properties as well as photocatalytic performance are investigated. XRD results show that initially, the Ni12P5 phase is formed, then by increasing the P/Ni molar ratio in the starting solution, product phase gradually transforms to the Ni2P phase. The results showed that crystalline phase and morphology of the nanostructures affects the catalytic performance of the samples. The best result for methylene blue dye removal was achieved for the sample with 35/1 phosphorus to nickel molar ratio which had pure Ni2P phase.