Without grain boundary defects, single crystal alloys possess special physical and chemical properties, e.g., as key materials for advanced aerospace engines and gas turbine engines, Ni-based single crystal superalloys have excellent high temperature performance. The characteristics of single crystal alloys determines they can only be obtained by means of solidification and therefore, the solidification process has enduring influence on microstructure, composition distribution and physical and chemical properties of single crystal alloys. For instance, as dendritic structure being the main characteristics, dendritic parameters and macro and micro element segregation of single crystal alloy are closely related with the solidification process parameters, such as solidification speed, temperature gradient and so on. Researches show that, the effects caused by gravity, such as buoyancy convection, sedimentation and hydrostatic pressure, etc., directly or indirectly affect the solidification process parameters, and become the important causes leading to the composition segregation and solidification defects. However, the effect of gravity cannot be effectively removed under conventional terrestrial condition, and it is difficult to clearly reveal the gravity effects in the process of solidification and their mechanisms. Hopefully, this problem will be solved under microgravity environments. Therefore, in recent years, taking advantage of space or simulated microgravity environments, the gravity effects on single crystal solidification behavior and their roles in the course of defects forming were studied by domestic and international scholars. These researches are of great significance for obtaining better single crystal solidification microstructure, avoiding the formation of solidification defects and improving the quality and performance of single crystal alloys. This paper reviews the recent domestic and international research progress on single crystal growth under microgravity and prospects for future researches.