Previous studies have demonstrated that the 1.38 microns water vapor absorption band is very efficient in detecting thin cirrus. In the preset study we report a practical algorithm to quantitatively retrieve cirrus bidirectional reflectance using two channels near 0.66 and 1.38 microns. The apparent reflectance of cirrus clouds at the 1.38 microns band is essentially the bidirectional reflectance of these clouds attenuated by the absorption of water vapor above cirrus clouds. The 0.66 microns radiance data measured from an airborne or satellite-borne radiometer contain both surface reflectance and cirrus reflectance. Although the apparent cirrus reflectance at the visible band is not free of the surface reflection, the scatter-plots BETWEEN 1.38 microns radiances and 0.66 microns radiances can be used to derive the transmittances of water vapor above cirrus clouds. The transmittances can be applied to scale the effect of water vapor absorption for the derivation of true cirrus reflectances in the visible spectral region. To illustrate the applicability of the present algorithm, results are shown for cirrus reflectance based on the retrievals from AVIRIS and MODIS data. The derived cirrus reflectance can be used to remove cirrus effects in visible channels. The spatially-averaged cirrus reflectances derived from MODIS data can be used to establish global cirrus climatology.