Abstract: With the purpose to improve the interface bonding force of carbon fibers (CFs) reinforced thermoplastic resin of poly(ether sulfone) (PES), a kind of new CFs type sizing agent called sulfonated poly(ether sulfone) (SPES) was prepared by PES sulfonated modification. The effect of SPES sizing agent and its mass fraction on interfacial properties of CFs/PES composites were studied. The results show that after sizing of SPES, the fiber fuzz amount decreases, the friction resistance is impoved, and it can also infer the chemical reaction between the group of —SO₃H on the SPES sample with trace reactive functional groups on surface of CFs groups from FTIR and XPS. SPES sizing agent plays a bridge role in the interface of CFs/PES composites and creates a better bonding between them. Interlaminar shear strength (ILSS) of CFs/PES composites improves most significantly than unsizing CFs/PES composites, increased by 24% when the concentration of sizing agent is 1wt%. The SEM photographs confirm a more closely combination between CFs and PES at this concentration. Dynamic mechanical thermal analysis (DMTA) also proves that the concentration of 1wt% SPES sizing agent improves the glass transition temperature of CFs/PES composites most significantly.