In this paper, we have compared the relative humidity (RH) retrieved from 183 GHz channel brightness temperature (BT) of AMSU-B (Advanced Microwave Sounding Unit-B) with those obtained from radiosondes in the Bay of Bengal during BOBMEX (Bay of Bengal Monsoon Experiment) and in the Arabian Sea during ARMEX (Arabian Sea Monsoon Experiment). The NCAR (National Center for Atmospheric Research) radiative transfer (RT) model has been used to obtain the relationship between layer-averaged RH and BT. With the present RT model, it has been found that the linear dependence of natural logarithm of layer-averaged RH on BT is valid, if the profile of humidity is invariant. However, the relative humidity profiles obtained during BOBMEX and ARMEX experiments showed a large variability in the vertical profiles of humidity in middle and upper troposphere. In such situation, the relation between layer-averaged RH and BT was found to be linear.