Studies were made on the photochemical and dark reactions of poly (vinyl cinnamate). The results obtained were as follows: 1. The phtochemical reaction of poly (vinyl cinnamate) is a one step reaction forming four-membered ring. This reaction scarecely occurs thermally because of the high activation energy in the reaction path although it occurs adiabatically in the photochemical process. 2. The four-memberd ring formed photochemically is stable for heating, but easily goes back to the cinnamoyl groups photochemically. 3. In the four electrons system formed by two cinnamoyl groups, an excitation of one electron leads to the formation of a four-membered ring. 4. It is considered that the dark reaction of poly (vinyl cinnamate) is a multi-stage radical reaction following the formation of a free radical.