Abstract: This paper studied the benzophenone-sensitized photocrosslinking reaction of 3,4-polyisoprene. It was found that when benzophenone-sensitized 3,4-polyisoprene was irradiated by 80 Whigh pressure mercury lamp, the photocrosslinking reaction could take place. The gel fraction and crosslinking density were dependent on the irradiation time, the amount of benzophenone used and molecular weight of 3,4-polyisoprene.By means of FTIR and C¹³NMR, it was confirmed that in the presence of benzophenone the photocrosslinking reaction of 3,4-polyisoprene is essentially 2+2 addition.