Fe–48.8wt.% Sn monotectic alloy was rapidly solidified both in a drop tube and with the glass fluxing method, and its phase separation characteristics and structural evolution are investigated. The regime of cooling rate of different size droplets in the drop tube is calculated from 7.0×10² to 2.8×10⁴ K s⁻¹ with a decrease of droplet diameter from 800 to 100 μm. The effect of Marangoni migration is more significant than that of Stokes motion with the decrease of gravitational acceleration, but it has a weak influence on the dispersion of the second liquid phase. The immiscibility gap is calculated and it shows that spinodal decomposition is difficult to occur because the necessary undercooling is quite large. Nucleation and growth show different characteristics when droplet size varies. Although the Sn-rich liquid phase separates from the undercooled melt, it cannot change significantly the composition of parent liquid phase, and the growth of terminal composition solid is suppressed, which is confirmed by the measured and calculated dendritic growth velocity by bulk undercooling experiments with the glass fluxing technique. Well-branched dendrites of the solid phase are difficult to form during monotectic transformation when droplets solidify in the drop tube owing to the very short solidification period.