Phototropin (phot) is a blue light dependent kinase involved in the light responses. The phot contains two LOV domains (LOV1 and LOV2) as light sensing modules and a kinase domain. In this article, we review their photoreactions studied by the transient grating (TG) method, which has been developed in our group. We have clarified the reaction dynamics including conformational change in protein part as well as intermolecular interaction change from the view point of diffusion coefficient change. Despite similarities in their structures and photochemistries of the chromophore, LOV1 and LOV2 showed distinctly different reactions in the protein moiety. The molecular mechanisms of their functioning and diversity of photoreactions are discussed.