Reflection by the thermal phase grating formed in methanol containing small Pd metal particles has been measured by using a degenerate four-wave mixing optics equipped with a nanosecond Nd:YAG laser and a streak camera. Although the integrated intensity of the reflection depended on the cubic power of the laser intensity the temporal profile of the reflection in a pulse duration showed the delayed response, the theoretical profile of which has been formulated by considering the formation of thermal phase grating. The theoretical calculation based on heat conduction equation was in good agreement to the experimental results with respect to the shape of the profile, the intensity, and the polarization dependence.