Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and a-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation. Droplets of Ni-31.4% Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum under cooling of 241 K (0.15 Tm) Calculations of the nucleation rates of L2 (Pb) and a-Ni are shown that L2 (Pb) and a-Ni are indicative of the increase in undercooling, the volume fraction of monotectic cells increases, ) phase acts as the leading nucleation phase during the monotectic transformation.