The solidified microstructure of immiscible alloys strongly depends on the nucleation, diffusional growth, especially the coalescence of the second phase droplets in the miscibility gap. A numerical model based on the discrete multi-particle approach was developed to simulate the nucleation and coalescence mode of the second phase droplets during the earth-based processing of immiscible alloys (in this case, the effect of gravity cannot be neglected). The cooling rate is the major factor influencing the coalescence mode. Under the super-rapid or rapid solidification condition (＞104 K/s), Brownian collision is the dominant coalescence mode. Marangoni collision becomes the dominant mode under the sub-rapid solidification condition (＞102 K/s). In the conventional slow cooling scope(101 K/s), Stokes collision becomes the dominant coalescence mode, correspondingly, leading to a serious phase segregation.