Raindrop size distribution (DSD) measurements were taken with an onboard OTT Particle Size Velocity (Parsivel) disdrometer over the western Pacific during a marine survey from June to July 2014. Three subregions named south western Pacific (SWP), west western Pacific (WWP) and north western Pacific (NWP) were separated for a comparative study of the variability of DSD. In addition to disdrometer data, FY2E, MODIS, NCEP FNL and radiosonde data sets are used to illustrate the dynamical and microphysical characteristics associated with summer season rainfall of western Pacific. The DSD characteristics of six different rain rates and two rain types (convective and stratiform) were studied. Histograms of normalized intercept parameter log₁₀(N_w) and mass-weighted mean diameter D_m indicated largest log₁₀(N_w) values in WWP while largest D_m values in SWP, and the convective clusters in three regions could be indentified between maritime-like and continental-like. The constrained relations between shape µ and slope Λ, N_w and D_m of gamma DSDs are derived. An inverse relation of the coefficients and exponents of Z-AR^b for convective rain were found in three regions. The R(Z_H, Z_DR) estimator is proved to be more accurate than Z–R relation algorithm. And the empirical relations between D_m and radar reflectivity factor in the Ku- and Ka-bands are also derived to improve the rainfall retrieval algorithms in the open sea of Pacific. Furthermore, the possible causative mechanisms for the significant DSD variability in three regions were investigated with respect to convective intensity, raindrop evaporation and other meteorological variables.