The effect of thermal exposure time on mechanical and thermal property behaviors of high density polyethylene (HDPE) used for plastic pallet under an outdoor environment condition has been investigated. As the thermal exposure time is increased, a proportional increase until 36 weeks in tensile strength is observed, while an increasing tendency in impact strength at the early stage of thermal degradation and then a drastic decrease after 27 weeks are shown. These results can be explained by the increases of chain entanglement and crystallinity at the early stage, followed by the increase of crosslinking density, chain scission and the decrease in chain mobility at the late stage of degradation, due to thermal oxidation. Microscopic observation using digital microscope indicates the increases of micro-crack and surface roughness as thermal oxidation makes a progress. Fourier transform infrared spectroscopy result suggests the increase of carbonyl index value on the surface of thermally exposed HDPE, indicating that the hydrocarbon groups on the HDPE surface undergo the thermal oxidation. The long-term life time of HDPE could be predicted by carrying out an accelerated life test using Arrhenius model. In addition, the lifetime equation of HDPE is suggested in order to estimate the failure time at various operating temperatures.