The epoxy oligomer was prepared to contain spiropyran in the side chain successfully. The rate of ring closure reaction of photochromic dye tethered in the side chain of the epoxy oligomer was controlled by virtue of [2p + 2p] photocycloaddition between the double bonds in the chalcone units and photopolymerization of epoxy group placed in the chain ends, which is expected to control the free volume surrounding photochromic moiety. The kinetic measurement was conducted to prove the improved photostability of the merocyanine chromophore that is colored species of spiropyran. In short, the cationic photopolymerization and photocrosslinking reaction were accompanied under illumination of the identical wavelength UV light.