Abstract: The failure in space environment of seal materials used in manned spacecraft has been analysed.Such effects as fragility increase of the sealant in low temperature and the surface of seal material in high temperature,permeability increase of seal material in vacuum environment and elasticity loss due to radiation are the main cause of failure in space environment.The contamination of the surface such as window glass from condensable material out of seal material in vacuum environment is another important phenomenon which must be considered in seal design.