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Frequency dependence of brightness temperature (T-B) in the range 6 - 21GHz is studied as a function of surface roughness, vegetation cover and soil moisture using theoretical models. Frequency dependence of T-B is plotted from Multi-channel Scanning Microwave Radiometer (MSMR) T-B data of Indian Remote Sensing Satellite (P-series) IRS-P4 for different target types such as ocean, desert, forest, agricultural and snow regions. MSMR responds to the variations of the target parameters; however, it is noticed that the T-B data at 18GHz deviate from the theoretically predicted trend. The effect of atmosphere ( water vapour) is taken into account in the theoretical trend. However, the frequency dependence of T-B plots from Scanning Multi- channel Microwave Radiometer (SMMR) of Nimbus-7 closely resemble the theoretical trend. This indicates that the relative calibration of 18GHz TB of IRS-P4 has some problem and needs to be looked into. Approximate offset values for each land type are computed and presented here. A general comparison of T-B of MSMR and SMMR indicates that T-B of MSMR is higher by 4K, which needs to be further investigated.