Abstract: Sizing agents are the key component to form the composite interface. Understanding the influence of sizing agents on the interfacial and mechanical properties of composites has important scientific value for the development of high-performance composites. The surface structures, interfacial properties and interlaminar fracture toughness of two high strength glass fiber fabric reinforced epoxy resin composites (GFRPs) with two different sizing agents were studied. The results show that one of the sizing agent has significant surface enrichment of the silane coupling agent, resulting in the increase in dynamic contact angle and the decrease in interfacial shear strength between fiber and resin. Contrarily, another sizing agent has less surface enrichment of silane coupling agent. The corresponding fiber has smaller dynamic contact angle and higher interfacial shear strength with the epoxy resin. The mode I and mode II fracture of the composites with these two sizing agents are based on the same mechanism, i.e., the interfacial debonding between resin and fiber. The interfacial continuity is reduced for the composite with the sizing agent existing higher surface enrichment of silane coupling agent. While for the composite with the sizing agent with less silane coupling agent surface enrichment, a continuous interface formed between the interphase and resin, resulting in good interfacial adhesion. The mode I and mode II interlaminar fracture toughness increased by 56.5% and 62.2% compared with these of the former composite, respectively.