It is well known that “positron e+” interacts with matter and forms a positronium (e+- e−) with the electrons of matter. Both the probability of the positronium formation and the lifetime of positronium are profoundly affected not only by the physical state of molecular substances, such as amount of defects, crystallinity, the size and other properties of crystals, but also by the presence of ions, radicals and excited species, produced in the system. Therefore, one can follow solid state reactions by the measurement of lifetime of positronium and the probability of the formation in the reaction system without any degradative analysis. By applying this method to the radiation-induced reaction in organic crystalline state, polymerization of several monomers such as acrylonitrile, methacrylonitrile, acetoaldehyde and N-vinyl-carbazole, in the crystalline state has been studied in our laboratory. In this paper, the radiation-induced solid state polymerizations N-vinyl-carbazole are reported.