During the period of 29–30 May 2001, the development and northeastward propagation of a series of mesoscale convective systems (MCSs) produced heavy rainfall with a maximum 6-h accumulated rainfall of 120 mm in southern Taiwan. Moisture budget analyses of the MCSs evolution associated with this heavy rain event was carried out by utilizing high-resolution numerical simulation results from the atmospheric part of the triply nested, nonhydrostatic Coupled Ocean/Atmospheric Mesoscale Prediction Systems (COAMPS). The control (CTRL) run experiment successfully simulated the synoptic environment, although it did not reproduce well the magnitude of the low-level horizontal moisture flux convergence and the precipitation distribution along the Meiyu front over the Taiwan Strait presumably due to insufficient data over the ocean. As QuikSCAT oceanic winds were taken into the assimilation cycles (QUIK run), the experiment reproduced better precipitation in terms of both amount and spatial distribution especially the precipitation over the ocean, and received higher equitable threat score for precipitation forecast. The MCSs evolution was also better simulated as compared to that of the CTRL run. Moisture budget analyses in the subcloud layer revealed that the grid-scale horizontal moisture flux convergence and upward vertical moisture flux divergence represented the major contribution for moisture transport in all stages of MCS evolution, and their values increased substantially during the convection development. The value of subgrid-scale vertical moisture flux convergence in the 5-km grid (mainly turbulence) was more than double that for convection developed over land than over the ocean. Relative contribution from disturbances under 5 km and cumulus convection were also examined by comparing the subgrid-scale moisture budget terms in the 15-km and 5-km grids, which showed that transport from disturbances under 5 km was dominating for both MCS development over the ocean and land. It was also found that surface evaporation played a relatively important role in the upward moisture transport processes before the development of MCS over the ocean as compared to that during the development of MCS over both land and the ocean. Contribution from orographic effect to rain associated with the MCS and vertical moisture flux convergence in the subcloud layer was discussed based on a flux model of orographic rain.