The short- and long-term creep behaviors of ultra-high-molecular-weight polyethylene (UHMWPE) systems (compression-molded UHMWPE sheets and self-reinforced UHMWPE composites) have been investigated. The short-term (30-120 min) creep experiment was conducted at a load of 1 MPa and a temperature range of 37-62 degrees C. Based on short-term creep data, the long-term creep behavior of UHMWPE systems at 1 MPa and 37 degrees C was predicted using time-temperature superposition and analytical formulas. Compared to actual long-term creep experiments of up to 110 days, the predicted creep values were found to well describe the creep properties of the materials. The creep behaviors of the UHMWPE systems were then evaluated for a creep time of longer than 10 years, and it was found that most creep deformation occurs in the early periods. The shift factors associated with time-temperature superposition were found to increase with increasing temperature, as per the Arrhenius equation. The effects of temperature, materials, and load on the shift factors could be explained by the classical free volume theory.