On the basis of the experimental studies on viscoelastic properties of poly vinyl alcohol (PVA) films at various relative humidity (RH) and temperature conditions by dynamic mechanical analysis (DMA), the influence of both temperature and RH on the glass transition are discussed and an improved property model is developed to relate the dynamic modulus to RH and temperature. The results indicate that (1) with increasing the RH, the storage modulus of PVA decrease remarkably, while both loss modulus and tan sharply increase to reach the peak and then markedly drops. The intensity of this variation is highly dependent upon temperature. (2) Moisture increase will cause the glass transition of PVA at isothermal condition and the transition point can be detected by glass transition relative humidity (RHg) that obtained by isothermal RH scans. (3) Similar to the relationship between T-g and RH, the RHg of PVA vary linearly with temperature. The state diagram of RHg versus temperatures is nearly consistent with that of T-g versus RH. (4)The present equation based on model of Mahieux and Reifsnider (Mahieux and Reifsnider, Polymer 2001, 42, 3281) can predict well the dynamic modulus of PVA at various RHs and temperatures. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3161-3167, 2013