When natural fiber-thermoplastic composites are used in long-term loading applications, investigating creep behavior is essential. The creep behavior of high-density polyethylene (HDPE)-based composites reinforced with four sizes of wood fibers (WFs) (120-80, 80-40, 40-20, and 20-10 mesh) was investigated. The instantaneous deformation and creep strain of all WF/HDPE composites increased at a fixed loading level when the temperature was increased incrementally from 25 to 85 degrees C. At a constant loading level, composites containing the larger-sized WFs had better creep resistance than those containing smaller-sized fibers at all measured temperatures. The creep properties of composites with smaller-sized WFs were more temperature-dependent than those with larger-sized WFs. Two creep models (Burger's model and Findley's power law model) were used to fit the measured creep data. A time-temperature superposition principle calculation was attempted for long-term creep prediction. The Findley model fitted the composite creep curves better than the four-element Burger's model. From the predicted creep response of the WF/HDPE composites, two groups of small fibers (120-80 and 80-40 mesh) had the lowest creep resistance over long periods of time at the reference temperature of 25 degrees C. The largest WFs (10-20 mesh) provided the best composite creep resistance. POLYM. ENG. SCI., 55:693-700, 2015. (c) 2014 Society of Plastics Engineers