A sputtered nanocrystalline coating on a coupon of the same K38G alloy was produced by magnetron sputtering. The effect of nanocrystallization on the hot corrosion behavior of K38G alloy with sulfate film deposit in air at 900degreesC was investigated. Surface and cross-sectional morphologies of the oxide formed were examined by SEM/EDX (scanning electron microscopy and energy-dispersive X-ray analysis) and EPMA (electron probe microanalysis). The results indicated that the mass gain of uncoated cast alloy decreased with the increase of the thickness of the sulfate film. Internal sulfidation occurred in the uncoated cast K38G alloy with 0.8 and 3.0mg/cm(2) of 75wt.%Na2SO4+ 25wt.%K2SO4. However, the mass gain for the coating increased with the increasing amount of sulfate deposit. In the case of the coating with a sulfate deposit of 0.8mg/cm(2), internal sulfidation was not observed. Although internal sulfides still formed for the coating with a sulfate deposit of 3.0mg/cm(2), the hot corrosion resistance of K38G alloy was clearly improved through nanocrystallization. The relevant hot corrosion mechanism is discussed.