The microstructure, chemical and phase composition of the hard Ni-P-Ti layer formed on the Ti-6Al-4V alloy after duplex surface treatment were investigated by light microscopy, X-ray diffraction, scanning electron microscopy and analytical/high-resolution transmission electron microscopy. These investigations showed that the improved mechanical and tribological properties of the surface-treated alloy were related to the presence of a multilayered microstructure containing several phases from the Ni-Ti-P-Al system.