A method to retrieve the mass-weighted mean raindrop diameter (<em>D<sub>m</sub></em>) from a space-borne Doppler radar is presented for Ku, Ka and W-band radars. The contribution of the air motion to the measured Doppler velocities which are the sum of the raindrop fall velocity and the vertical velocity of the atmosphere (<em>V<sub>air</sub></em>), was removed by a physically-based algorithm. The attenuation corrected reflectivity factors, the specific attenuation and the Doppler velocity are used as input in the algorithm to estimate <em>D<sub>m</sub></em> from the theoretical relationships among those values. For Ku-band, the effects of air motion were well removed, whereas the effects of DSD were difficult to remove due to the Rayleigh scattering regime. The latter effects were reduced by using a technique to determine an appropriate DSD by using the dependence of <em>Z</em>-<em>R</em> relationship on <em>D<sub>m</sub></em>. For W-band, modified algorithm were developed to estimate <em>D<sub>m</sub></em>. The validations of the retrieval method were made using simulated rain drop size distributions. A Monte Carlo model was used to evolves DSD by coalescence and breakup in a convective rain. Uncertainties in the retrieved <em>D<sub>m</sub></em> arising from the measurement errors were examined. The validation results show good agreement between the estimated <em>D<sub>m</sub></em> and the model values calculated from the simulated raindrop size distributions.