Phototropins are blue-light-sensitive photoreceptor proteins in plants. Phototropins consist of two LOV (light, oxygen, and voltage sensor) domains (LOV1 and LOV2) that undergo photochemical reactions. Although the photochemical reaction of the LOV2 domain has been investigated extensively, the reaction of the LOV1 domain remains unresolved. In this study, the reactions of the Arabidopsis phototropin 2 LOV1 (phot2LOV1) domain were revealed by the transient grating (TG) method. The TG signal showed a significant diffusion coefficient (D) change upon photoexcitation. This change was sensitive to the protein concentration and the observation time range. These observations were explained by assuming that there are reactive and nonreactive forms, and the fraction of these species is concentration dependent. From the concentration dependence of the dynamics, the monomer was found to form a dimer; however, the dimer does not exhibit an observable reaction. In the dark state, both species were in equilibrium and were not distinguishable spectroscopically. For the LOV1 domain with the hinge domain, the reaction scheme was the same as the LOV1 domain sample, but the D change was affected by the presence of the hinge region. This observation suggested that the hinge region undergoes a conformational change during the photoreaction.