The bulk polymerization of styrene initiated by AIBN which was sensitized by PVN could

be effected by magnetic field under irradiation with 320 nm monochromic light. The molecular weight distribution of polystyrene is dependent on the concentration and molecular weight of PVN. It was confirmed by experiments that the critical molecular weight and concentration of PVN which effected on the polymerization are 1.7×103 and 1.62×10-3 mol/l respectively. The cause which is responsible for these phenomena was discussed qualitatively.