The composition dependence of glass-forming ability (GFA) of Ni-P binary alloys was systematically examined by fabricating ribbons of different thicknesses, and the microhardness of the glassy ribbon was measured. The eutectic alloy Ni80.4P19.6 has the best GFA and the highest microhardness in glassy state. As the alloy composition is deviated from the eutectic composition, both GFA and microhardness decrease, accompanied with an increasing full width at half maximum of the main broad peak in the X-ray diffraction spectrum of the glassy ribbon. All these results indicate a close correlation among microstructure, GFA and microhardness for the metallic glass. (C) 2012 Elsevier Ltd. All rights reserved.