With a view to surface-modification for better adhesion between fiber and matrix in carbon fiber(CF)/high density polyethylene(HDPE) composites, acrylamide(AAm) possessing functional groups available for chemical bonding with CF was graft-polymerized onto the same (stem) Polymer as the matrix PE to give HDPE-g-PAAm by the inverse emulsion graft polymerization technique. Graft polymers(GP) synthesized at different conditions of stem polymer molecular weight, concentration of AAm aqueous solution, and contents of emulsifier and initiator were classified and separated into 3 types according to the PAAm content in a GP, i.e., toluen-soluble(TSGP), methanol-dispersible(MDGP), and water-dispersible(WDGP) in the increasing order of PAAm content. In HDPE composites reinforced with CF surface-treated using the GP thus obtained, the effects of such factors as the concentrations of emulsifier and initiator, M_w of the stem polymer, and the type and adsorption amount of GP on the impact strength of laminated composites were investigated for different types of coupling agents. In addition, the influence of the cross-linking of matrix polymer on impact strength of CF/HDPE composite was studied by means of either separate use of preoxide and coagent or their combination. Finally, the "suspension adsorption method" for preparing thermoplastic prepregs based on toluene suspension of PE with filament tows of CF dipped into it was developed, which turned out to give the uniform dispersion of fibers within the matrix resin as well as the relatively easy control of fiber content in a composite.