The impact of the three dimenstion (3D) effect on the passive microwave rainfall estimations is examined by synthetic retrievals employing a Bayesian methodology. The results showed that the uncertainty in the rainfall estimations due to the 3D effect depended on the viewing directions considered in the a-priori information. It was also found that taking more viewing angles or the azimuth angles in the a-priori information into consideration tended to moderate the retrieval difference that resulted from the different viewing directions. In addition, the retrieval uncertainty related to the 3D effect appeared to be more significant for heavy rainfall cases with large amounts of ice particles, as expected. We additionally performed the retrieval experiments with the databases constructed with the one dimensional slant path (1D-SP) calculation. In general, the 3D model-based experiments slightly outperformed the 1D-SP model-based experiments. However, the 1D-SP model may be considered as an alternative model for the full 3D radiative transfer model with limited computing resources and a required level of the retrieval accuracy.