Nickel phosphide (Ni2P) nanoparticles (NPs) with different sizes were synthesized via thermal decomposition of bis(triphenylphosphine)nickel dichloride precursor in the presence of oleylamine. The size of the as-synthesized Ni2P NPs could easily be controlled by increasing the reaction temperature from 300 to 340 degrees C. The structure and morphology were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N-2 adsorption-desorption and X-ray photoelectron spectroscopy (XPS). Then the influences of the size of the Ni2P NPs on the magnetic and electrocatalytic properties were investigated systematically. The results indicate that the as-synthesized Ni2P NPs exhibit ferromagnetic characteristic at 5 K. The Ni2P NPs with small size exhibit superparamagnetism and the larger size exhibit ferromagnetic characteristic at 300 K. The blocking temperature, saturation magnetization, remanent magnetization and coercivity increased significantly with the increase of size of Ni2P NPs, indicating the strong size effect of Ni2P NPs for magnetic properties. Electrochemical tests indicate that the catalytic activity can be enhanced by decreasing the size of Ni2P NPs. Due to the larger electrochemical active surface area and higher electrical conductivity, the Ni2P NPs with small size exhibit higher electrocatalytic activity. This work suggests that the size of Ni2P NPs is an important factor to affect the magnetic and electrocatalytic properties. (C) 2016 Elsevier B.V. All rights reserved.