The novel photoreactive crystal, 4-(4-(2-(ethoxycarbonyl)vinyl)-cinnamoylamino)benzoic acid, (ECAB) was prepared and its photoreactivity evaluated. By introducing an amide group into a p-phenylenediacrylic acid derivative, a "tape" structure network was constructed. On irradiation the crystals were converted into dimers quantitatively. Characterization of the dimer and the results of X-ray crystallographic analysis showed that photodimerization proceeds with selective cycloaddition of olefinic bonds on the "amide side" only and accompanied by trans-cis isomerization of an olefinic bond on the "ester side".