The relation of density of polypropylene without and with various additive(carbon black, titanium dioxide, phthalocyanine blue and helindon orange)versus its isothermal crystallization temperature, annealing period and irradiation was examined in this paper. The change of density was affected by the crystalline degree and the spherulite diameter of polypropylene samples.The increasing value of irradiated polypropylene density depended on its crystallization temperature and annealing period, which increased up to a maximum value for each given crystallization temperature, due to the crystallization of smaller molecules, which were formed by radiation chian scission. The densities were change very less after irradiation without annealing.