The sensitivity and stability of an optical heterodyne detected transient grating (OHD-TG) method without an active phase control system was examined, and the system was used to separate the phase and amplitude gratings. From the absorptive component at 633 nm after the photoexcitation of a photochromic molecule (a spiropyran molecule), the diffusion constant of only the merocyanine form is selectively determined. The phase grating component consists of both the spiro and merocyanine contributions. However, since the contribution of the merocyanine form is still predominant over the spiro form in the phase grating signal, the diffusion process of the spiro form cannot be observed clearly. By adjusting the phase between the local oscillator and probe fields, the contribution of the merocyanine form can be selectively reduced and the diffusion constant of the spiro form is successfully measured. The result shows that the selective reduction of one species by the OHD-TG technique has a great advantage for studying photochemical reactions.