Abstract: Hydroxyl- and amino- functionalized carbon fibers (CF―OH and CF―NH₂) were prepared by surface oxidation with an acid mixture (H₂SO₄∶HNO₃ of 3∶1 v/v) followed by grafting with ethylenediamine. The functionalized CFs were sized with a sulfonated poly (ether ether ketone) (SPEEK) sizing agent to prepare CF―OH―SPEEK and CF―NH₂―SPEEK materials. The effect of surface functionalization on the surface properties of the CFs and their interfacial properties in PEEK matrix composites were investigated. Results showed that the content of polar functional groups and wettability of the CFs increased significantly after surface functionalization. Chemical reactions between the modified CFs and the sizing agent, improved the interfacial adhesion between them. The interfacial shear strengths of CF―OH―SPEEK and CF―NH₂―SPEEK reinforced PEEK matrix composites were increased by 6.2% and 14.0%, respectively, compared with that of the composites reinforced with desized-SPEEK CFs. The surface functionalization helps improve the interfacial adhesion of thermoplastic-coated CF/PEEK composites.