Abstract: The brightness temperatures of IR spectral radiation of cirrus atmosphere with different cirrus altitude were computed by a combined atmospheric radiative transfer (CART) model. The effects of cirrus altitude on brightness temperature spectra of different IR bands and on the retrieval of optical thickness and effective size of cirrus were analyzed. The results show that the variation of IR brightness temperature with cirrus altitude in the atmospheric window region is consistent with the atmospheric temperature profile below the tropopause. The slope of the brightness temperature from 790 cm-1 to 960 cm-1 bands increases with the altitude of cirrus clouds. Variation of brightness temperature difference between 900 cm-1 and 1 231 cm-1 bands to cirrus altitude is sensitive to thin cirrus and small effective size. Variation of brightness temperature difference between 900 cm-1and 1 559 cm-1 bands to cirrus altitude is basically not dependent on effective size and optical thickness for thick cirrus. Thus, the effects of cirrus altitude on the remote sensing of optical quantitation of cirrus parameters should be considered.