One of the main goals of the National Aeronautics and Space Administration (NASA) Global Precipitation Measurement (GPM) mission is to retrieve parameters of the raindrop size distribution (DSD) globally. As a standard product of the dual-frequency Precipitation Radar (DPR) on board GPM Core Satellite, the mass-weighted mean diameter, Dm, and the normalized intercept parameter, Nw, are estimated in three dimensions at the resolution of the radar. These are two parameters of the three-parameter gamma model DSD adopted by the GPM algorithms. This study investigates the accuracy of the Dm retrieval through a comparative study of C-band ground radars (GR) and GPM products over Italy. The reliability of ground reference is tested by using two different approaches to estimate Dm. The results show good agreement between the ground based and space-borne derived Dm with an absolute bias being generally lower than 0.5 mm over land in stratiform precipitation for the DPR algorithm, and the combined DPR-GMI (GPM Microwave Imager) algorithm. For the DPR-GMI algorithm, the good agreement extends to convective precipitation as well. Estimates of Dm from the DPR High Sensitivity Ka-band (HS) data show slightly worse results. A sensitivity study indicates that the accuracy of the Dm estimation is independent of the height above surface (not shown) and the distance from the ground radar. On the other hand, a non-uniform precipitation pattern (interpreted both as high variability and as a patchy spatial distribution) within the DPR footprint is usually associated with a significant error in the DPR-derived estimate of Dm.