Objective To investigate the main component of the domestic aramid fiber ( DAF-III) sizing agents and its effect on the interfacial properties of composites. Methods The fibers were extracted by acetone for a certain period of time. The extracts and fibers before and after extraction were characterized. The DAF-III reinforced polytriazole ( PTA) composites were prepared, and the mechanical properties were tested. Results The results showed that the main molecular structure of domestic DAF-III and surface sizing agent were polyamide benzimidazole and aliphatic ester oligomer, respectively. The majority of the sizing agent was removed and plenty of grooves were formed along the axial direction on the exposed fiber consequently. The mechanical properties ( ILSS and flexural strength) of desized fiber reinforced PTA resin composite were increased by 181. 2% and 56. 20% , comparing with those of the virgin sample. Conclusion The sizing agent had a remarkable impact on both fibers and mechanical properties of DAF-III reinforced PTA composite. The fiber interacted with PTA through the mechanical interlocking and hydrogen bond.