The photochemical graft copolymerization of isoprene on the surface of tetrafluoroethylene-propylene copolymer was effected by external magnetic field. The graft ratio of the system using benzophenone as initiator in the presence of 1.2 Tesla of external magnetic field is more than three times than in the absence of magne tic field. The 3

4-polymerized contents in po-lyisoprene were increased greatly. The causes which led to above-mentioned phenomena were discussed.