Moisture sorption and desorption behavior of polymer materials (ASA, ABS, PMMA, PC) for automotive exterior lamp (headlamp and rearlamp) has been investigated in various temperature conditions. The variations of the moisture sorption and desoption have been represented following the theoretical model of the Fick’s second law of diffusion. In order to understand the dimensional change during the moisture sorption process, the change in length of each sample was studied. The length of polymeric materials increased as equilibrium water content increased. The moisture absorbed polymer materials provided a 50% decrement of the coefficient of thermal expansion, seemingly by swelling the polymer materials under atmosphere condition. The parameters of the diffusion process such as diffusion coefficients and activation energy were identified and modeled by both experimental and numerical approaches. The diffusion coefficients of moisture in desorption process were measured and exhibited a linear relationship with the inverse temperature. The results of this study were utilized as prediction of the humidity change in the automotive lamp through the relationship between humidity and diffusion coefficient.