To research the effects of temperature and mechanical stresses on the dielectric breakdown strength( DBS) of Glass Fiber /Epoxy ( GF /EP) composites，the AC breakdown test has been carried out on the GF /EPcomposites under different temperatures、different stress magnitude and different style of the stresses，the effects oftemperature and mechanical stresses on the dielectric strength have been analyzed according to the dielectricbreakdown theory. The results show that: the dielectric strength of GF /EP composites decrease with increasingtensile stress under certain temperature; the dielectric strength increase at first with increasing bending stress，reach a maximum and indicate a decreasing tendency thereafter; compared to room temperature，the dielectricstrength of GF /EP composites have been increased in different level under certain stress. The results provideimportant experimental basis for GF /EP composites in the practical application.