Annealing-induced structural transition from amorphous to nanocrystalline states is believed to have a significant effect on the wettability of metallic glass alloys. In this study, an amorphous Ni80P20 alloy was pre-annealed at various temperatures to yield different structures and then wetted by a eutectic Sn-Bi solder at 473 K in a high vacuum using a dispensed sessile drop method. The results show that the structural relaxation and primary crystallization in the amorphous substrates greatly deteriorate the wettability while large-scale eutectic crystallization and the growth of nanocrystallites after high-temperature annealing improve it primarily due to the formation of a precursor film ahead of triple line. Moreover, the amorphous Ni-P substrates are more reactive with the molten Sn-Bi solder than the crystallized ones. The interfacial reaction yields Ni3Sn4 and a P-rich crystalline layer consisting mainly of Ni3P and Ni2SnP intermetallics. The presence of this layer dramatically retards the subsequent growth of the Ni3Sn4 phase.