The solidification behavior of a Sn-20% (mass fraction) Ni peritectic alloy in conditions of ordinary gravity and microgravity were comparatively studied by using a 50-meter-high drop tube. The solidified microstructure of the alloy was observed by optical metalloscopy (OM), the percentages of residual primary phase, peritectic phase and final solidification phase in the samples were counted by IPP (Image Pro Plus) software, and the solute distribution and phase composition in the samples were determined by means of energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD). The results show that the solidification process of Sn-20% Ni may involve the primary phase nucleation at solid-liquid interface front, dendrite growth and peritectic reaction. Gravity has significant effect on both the primary phase formation and peritectic reaction, resulting in significant difference in the partition and distribution of phases as well as the distribution of alloying elements obtained in the conditions of microgravity and ordinary gravity respectively. The amount of residual primary phase and the total amount of residual primary phase plus peritectic phase in ordinary gravity condition is always lower than those in microgravity condition, while the quantity of peritectic phase is always higher. In addition, the distribution of solute element in the samples are basically consistent with those of the total amount of residual primary phase plus peritectic phase. These results indicate that the microgravity environment is favorable to the formation and growth of primary phase of the Sn-20% Ni alloy, in the contrary, the gravity environment promotes peritectic reaction, which is related to buoyancy convection and crystal nucleus deposition induced by gravity.