Polyacrylonitrile graft copolymers containing poly(ethylene oxide) and di methylamine side chains were synthesized photochemically. Interfacial free energy, NMR spectra and in vitro platelet adhesion were examined for the graft copolymers with various compositions. In vivo tests for the nonthrombogenicity of the polymers were also carried out. The graft copolymers of moderate or high poly(ethylene oxide) content were least thrombogenic when dimethylamine content was low, and most thrombogenic when dimethylamine content was high. The results were discussed in terms of the physical properties of the graft copolymers.