In the crystals of phthalic acid trans-cinnamamide (1:2), two molecules of cinnamamide are connected with phthalic acid via a hydrogen bond to form a three-component assembly. The C=C double bonds of two cinnamamides take a criss-crossed arrangement with a center-to-center distance of 4.252(3) Angstrom and with a twist angle of 97.1(4)degrees. After a partial single-crystal-to-single-crystal transformation, X-ray analysis revealed the beta -type photodimer of cinnamamide in the assembly at 13.2(3)% conversion. This suggests that the pedal-like conformational change occurred for one of the cinnamamide molecules and the C=C bond of the other cinnamamide moved approximately parallel to form a cyclobutane ring.