Changes in the mechanical properties of Kevlar/epoxy and glass/epoxy composites with increasing immerision time have been studied. Kevlar/epoxy composite showed poorer mechanical properties than glass/epoxy composite with increasing immersion time in water. This is because Kevlar/epoxy absorbed more water than glass/epoxy composite did. Effect of thermal fatigue on the mechanical properties of kevlar/Spectra hybrid composite has also been studied. Strength of hybrid composite showed a sharp decrease with increasing thermal fatigue cycle due to the decrease in interfacial strength between fiber and matrix.