The effect of various environmental conditions on the durability in E-glass fiber/vinylester resin composites has been investigated using tensile test specimen of strand type. Tensile strength decreased with increasing immersion time in all environmental conditions. The extent of decrease was more severe in the case of alkaline condition than deionized water condition regardless of temperature. We found that the decrease in tensile strength during the immersion time in various environment conditions was mainly caused by the degradation of the interface between glass fiber and vinylester resin and damage of glass fiber surface.