The kinetics of gel formation in the radiation induced crosslinking reaction of isotactic polypropylene (i-PP) and its blend system was studied. The relationship between sol fraction (s) and irradiation dose (R) in radiation crosslinking of PP well agrees with assumption fracture density of polymer is proportional to R,β, where β is a parameter depending on the structure of a polymer, and good linear relation between ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650057&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650059&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650058&type=, and R,β, of PP was obtained. In the case of PP/HDPE and PP/EPDM blends, based on the characteristics of phase structure and the interaction among the components, which were studied by scanning electronics microscope and dynamic viscoelastometer, the R,β, of i-PP blend can be modified as: R,β,=Σ x,i,R,βi, x,i, and β,i, are weight fraction and β value of i component in blend respectively. Using this modified R,β, good relations between ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650048&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650063&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650061&type=, and R,β, of PP/HDPE or PP/EPDM blends in radiation crosslinking reaction were also obtained.