In order to investigate the effect of surface treatment of fiber on the crystallization and interfacial morphology in the carbon fiber/high density polyethylene (CF/HDPE) composite system, carbon fibers were surface-modified with two types of coupling agents, water-dispersible (WDGP) and toluene-soluble(TSGP) HDPE-g-PAAm copolymer prepared by the inverse emulsion graft polymerization. Composite films containing unidirectional CF's were prepared by press molding through the isothermal crystallization process. The change in crystallization temperature(T_c), transcrystallinity, and interfacial bonding state in HDPE composites reinforced with modified carbon fibers were analyzed in terms of the type(or the content of PAAm in a graft copolymer) of the coupling agent by using DSC, polarizing microscope, FTIR, SEM, etc. The surface-treated CF/PE composite systems were found to exhibit better developed interfacial crystal(transcrystal) structure and somewhere incresed T_c compared to the untreated CF/PE system. As the content of the grafted PAAm in a compling agent increased, the fraction of the hydrogen bonding present in a composite system increased, thereby enhancing the interfacial bond strength between fiber and matrix, as confirmed by observation on the tensile fracture surface of the composite. It was also revealed that the treated composite systems had larger transverse tensile moduli than the untreated case probably due to the increased interfacial adhesion, this tendency being more pronounced in the case of the WDGP coupling agent.