We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state We attributed the fluorescence spectral evolution of the microcrystal under UV irradiation to the photoinduced trans-to-cis isomerization The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation In addition the [2 2] topochemical photocyclodimer was observed as another photoproduct It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal sui lace and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal