Thermal diffusivity has been investigated in a manganite thin film La0.6Sr0.4MnO3 by means of transient grating(TG) technique at room temperature. A new method, which is generalized to two-layered samples of the thin film deposited on a semi-infinite substrate, is established to fit the TG signals. The thermal diffusivity of the La0.6Sr0.4MnOs thin film with a thickness of 200nm on an MgO (100) substrate is determined to be 0.92mm2/s,which is slightly smaller than that of the single crystal sample (1 mm2/s).