Monodispersed nickel phosphide nanocrystals (NCs) with different phases (Ni₁₂P₅, Ni₂P and Ni₅P₄) were synthesized via the thermal decomposition approach using nickel acetylacetonate as the nickel source, trioctylphosphine as the phosphorus source and oleylamine in 1-octadecene as the reductant. The phases of the as-synthesized nickel phosphide NCs could easily be controlled by changing the P : Ni precursor ratio. The structure and morphology of the as-synthesized nickel phosphide NCs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and N₂ adsorption–desorption. A formation mechanism for the as-synthesized nickel phosphide NCs was proposed. We further studied the influence of the phase of the nickel phosphide NCs on the electrocatalytic properties for the hydrogen evolution reaction (HER). All phases showed good catalytic properties, and the Ni₅P₄ NCs with a solid structure exhibited higher catalytic activity than the Ni₁₂P₅ and Ni₂P NCs. This superior catalytic activity is attributed to the higher positive charge of Ni and a stronger ensemble effect of P in Ni₅P₄ NCs. This study demonstrates that the crystalline phase is important for affecting the electrocatalytic properties.