We have found that DMT would act as active initiator for the photopolymeriza-tion of aerylonitrile．The sensitization eft ect of various aromatic tertiary amines on the photoplymerization of AN has been studied．The electro．positive group on the benzene ring will increase the activity of the amine while the electro—negative group will decrease it．The activities of various amines have been found in the following deereas-ing order：N，N-dimethyltoluidine(DMT)>N，N-d'methyl．J3-hydroxyethyrItoluidine(DHET)> N，N-dimethylaniline(DMA)>N，N．dimethylamino ben zaldehyde(DMB)>N，N． dimethyl-p-nitroaniline (DNA)．The mechanism of photoplymerization of AN initiated with aromatic tertiary amine belongs to the free radical type in nature，and the rate of polymerization is proportional to the 0.66th power of concentration of amine． Within the range of 10-2---10-4 M in the concentration of amine the reciprocal of average degree of poly-merization is proportional to the 0.5th power of eoneentration of amine．The tentative mechanism of initiation has been proposed．The formation of initiating radical speeies may be ascribed to the decomposition of the excited charge transfer complex (exciplex) from the excited state of aromatic tertiary amine with the monomer molecule of aerylonitrile．