The radiation-induced crosslinking of siloxane copolymers with various contents of vinyl group has been studied. It was found that the existance of vinyl groups in the copolymer macromolecules could reduce the gelation dose and hence increases the G value of crosslinking.,The variation of the content of vinyl groups in sol fraction of irradiated samples with dose was followed by NMR. It has been shown that they remain unchanged with radiation dose. Such a result indicates that the vinyl group is very sensitive to radiation-induced crosslinking. The effect of vinyl group on the enhancement of crosslinking could be well understood.,When a semi-crystalline copolymer at -78℃ was irradiated such an effect was much pronounced. It results from concentrating of vinyl group in the amorphous region so that greater probability exists in reaction with radical before they decay with a process without crosslinking formation.,The vinyl group in siloxane copolymer could also influence the deviation from the linear relationship given by Charlesby and Pinner. The effect of vinyl content on the relationship between crosslinking density and radiation dose is expected to be in a similar situations.