Beamfilling error is one of the main error sources for microwave oceanic rainfall retrieval. An accurate beamfilling correction can improve the rainfall retrieval accuracy significantly. Quantitative understanding of the uncertainty of the Beamfilling Correction Factor (BCF) is very important for the understanding of the accuracy of microwave passive rainfall retrieval. Refinement of the calculation of the BCF and the estimation of BCF uncertainty are the main purposes of this thesis. The characteristic of rainfall distribution is investigated. Quantitative understanding of the statistical characteristics of rainfall distribution provides an indication of the beamfilling error and the uncertainty of BCF in many ways. Some refinements to the traditional BCF calculation algorithm are provided in this thesis. Scattering is included in the new algorithm. Also the BCF calculation only considers the cases within the useful dynamic range. These refinements make the BCF calculation closer to how it is used in the retrieval algorithm. The BCF based on the new algorithm should be more accurate. The global BCF uncertainty and the local BCF uncertainty are estimated using the available A/C radar data. The results show that the uncertainty of BCF is much smaller than expected, and also show that the BCF derived from a specific set of data can be used globally.