A parameter named Normalized Difference Fog Index (NDFI) is proposed to detect land radiation fog based on analyzing the spectral character of fog and cloud in visible, short infrared and middle  infrared wave band by employing the Streamer radiative transfer model and the MODIS data. Threshold is set according to the histogram. It is demonstrated by two cases over mainland China in winter that this NDFI parameter could be applied to perform land radiance fog detection with high accuracy.