The effect of hydrolysis of polycarbonate on tensile behavior were investigated. The specimens were exposed in a chamber at constant room temperature and water for 300 hours, and measurements were made on the variation of weight and tensile properties. In addition, the increase and decrease of the carbonate part was measured with an ultraviolet spectroscopy. The results are as follows: It was confirmed that it was effective to the examination of the influence of the diffusion of water with an overlay castfilm. It has been understood to the diffusion of water from the surface layer has diffused at the same time by about 6μm, and it has diffused gradually when it is deeper than 6μm. The change in the tensile behavior by the hydrolysis was able to be confirmed by observing the carbonate part of the polycarbonate. It became clear, the break strain has decreased because the carbonate part is taken apart by the hydrolysis. Afterwards, the break strain steadies by constructing a bridge, and the yield stress increases.