Abstract: The effects of antioxidants, anti-hydrolytic agents and heat stabilizers on the thermal aging properties of PCL/white poplar fiber (WF) composites were investigated by mechanical property measurements, infrared spectroscopy, SEM observation and XRD analysis. The results indicated that the addition of antioxidant (BHA), anti-hydrolytic agent and heat stabilizers could effectively retard the thermal aging of PCL/WF composites. The optimum contents of additives could be determined as 4 wt % of antioxidant, 4.5 wt % of anti-hydrolytic agent and 2 wt % of heat stabilizer. A synergistic anti-aging effect was achieved in the presence of more than one anti-aging agent.