Two origins of the refractive index variation after depositing heat energy to a fluid are separately investigated by using the transient grating (TG) method; the variations are caused by a density fluctuation and a temperature fluctuation with a constant density. Although the relative contribution of the temperature rise (constant density) component is small in the total refractive index change, a precise measurement of the TG signal enables us to determine the magnitude and it is found that they are as large as 3%-6% in magnitude of dn/dT except water, in which the relative contribution strongly depends on temperature. The origin of the temperature rise component is interpreted in terms of the interaction induced polarizability change of the fluid molecules. An application of this component to the ultrafast detection of the photothermal techniques is discussed.