MST radars at Koto Tabang, Sumatra and Gadanki, south India, are used to estimate vertical profiles of raindrop size distribution (DSD) to improve the model for radar rainfall remote sensing and to obtain information of micro-physical processes of raindrops during their fall. The methodology employed is a non-linear least-square estimation of a set of gamma DSD model parameters including a "normalized 6th moment", conceptually similar to "normalized N0". In this presentation, the methodology of the DSD estimation is discussed in comparison with other DSD parameterization. Several event analyses are presented to investigate the dependence of vertical DSD profile on climatological regimes and cloud microphysics which are different from one another at the two tropical sites. The result may also be useful to model vertical DSD profiles at other tropical regions.