Ni-P-W alloy deposit on the mild steel substrate was obtained by the electroless process. Various thermal heat-treatment temperatures from 350 to 600 degreesC for 4 h were performed on the coating to evaluate the effect of crystallization behavior and grain size on the mechanical properties. The microstructure at as-deposited and heat-treatment states were investigated by the X-ray diffraction (XRD) and transmission electron microscope (TEM). The hardness at all depths of the coating on the substrate could be acquired by the nanoindentation. The Ni-P-W coating showed an amorphous structure in the as-deposited state and exhibited a relatively low hardness of approximately 6.8 GPa. As temperature was raised to 380 degreesC, the hardness was slightly increased to 8.7 GPa due to the partial precipitation of Ni and Ni3P in the amorphous matrix. On heating to 500 degreesC for 4 h, the hardness reached the maximum value of 12.3 GPa with a grain size of 33.1 nm and followed by gradual degradation above 500 degreesC. The Young modulus of the amorphous Ni-P-W coating showed a relatively low value of 195 GPa. However, for increasing the heat-treatment temperature accompanied with the grain growth, the Young modulus was enhanced to the maximum of 233 GPa and then decreased gradually with temperature. (C) 2004 Elsevier B.V. All rights reserved.