In situ characterization of the morphology of organic thin films was newly attempted by using total reflection X-ray analysis (TRXA). Due to cover a substrate with organic molecules during deposition, the intensity of total reflection fluorescence X-rays from a substrate surface decreases. It is expected that the decrement depends on the morphology of thin films. Fullerene (C60) thin films on the silver (Ag) substrates were formed at the substrate temperatures of 17, 50 and 100 degrees C. From a result of in situ observation for the fluorescence Xrays of Ag substrates by TRXA, the intensity steeply decreased at 17 degrees C and was saturated at 50 and 100 degrees C. It was confirmed that the decay curve indicates the ratio of the coverage of C60 and the growth mode of island or layer-by-layer.