Four different Ni-Pb binary monotectic alloys have been rapidly solidified during free fall in drop tube. With the decrease of droplet diameter, the primary α-Ni dendrite in Ni-20%Pb hypomonotectic droplets shows a morphology transition of "coarse dendrite→equiaxed dendrite→vermiculate dendrite". The solidification microstructure of Ni-31.4%Pb monotectic alloy is mainly composed of monotectic cells, the volume fraction of which increases with the decrease of droplet diameter. Liquid phase separation has happened during the rapid solidification of both Ni-50%Pb and Ni-60%Pb hypermonotectic alloys. The grain size of L₂(Pb) phase in Ni-50%Pb alloy increases and then decreases when the droplet diameter decreases. The microstructures of Ni-60%Pb alloy droplets display a "core-shell→dendrite" transformation as the droplet diameter becomes smaller than 400 μm. Theoretical analyses of Borelius model, Gibbs-Thomson equation and Lifshitz-Slyozov-Wagner(LSW) theory reveal that alloy composition, phase separation time and volume fraction of minor phase play important roles in the formation of core-shell microstructure.