The effect of various environmental conditions on durability of E-glass fiber/vinylester resin composites has been investigated with tensile test specimen of strand type. The durability test method performed by strand type specimen was more convenient and reliable than other conventional test method. The weight gain of E-glass fiber reinforced composites increased with the immersion time in both water and alkali solutions, and the weight gains at 80°C were a little greater than ones at 20°C. The tensile strength decreased with the immersion time in all aqueous solution, and the tensile strength in 80°C of alkaline solution decreased most rapidly. The decrement of tensile strength in various environmental conditions was mainly caused by the degradation of interface and damage of glass fiber surface.