The radiative forcing of cirrus clouds, one of the most uncertain components in climate study, is associated with two parameters, the effective size of ice crystals within cirrus and the corresponding cloud optical thickness. We introduce a method to simultaneously retrieve ice crystal effective size and cirrus cloud optical thickness using 1.38- and 1.88-µm cirrus reflectance from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data. Atmospheric and surface effects are removed from this data to obtain isolated cirrus reflectances at these two bands. Forward numerical computations are carried out to establish the correlation between cloud reflectance and the pair of effective size and optical thickness. An algorithm based on a look-up table approach to derive the effective size and optical thickness of cirrus clouds has been implemented on a pixel-by-pixel basis. The applicability of this method is demonstrated using several independent AVIRIS images.