Spectrum attenuation changes for MCVD optical fiber exposed to a hydrogen atmosphere at 100, 130, 150 and 180℃ are measured. The loss increases at 1.24 μm and 1.41μm due to molecular hydrogen and hydroxyl respectively are investigated. The empirical formula for time and temperature dependence of the loss increase are obtained from the experimental results. The loss increase at 1.3 μm and 1.55μm, at room temperature are estimated.