A highly developed industry and a large population density have turned the western plain of Taiwan into a mega-suburb with many cities and small towns and countless factories, and roads. As a result, the western plain is experiencing a regional heat-island effect. The MM5 mesoscale model was conducted in order to study and evaluate the impacts of the heat-island effect on regional weather, including thunderstorms, over Taiwan. According to land use data provided by the US Geological Survey (USGS), we assumed three different urban sizes in the simulation study to theoretically evaluate the impact of urbanization on the precipitation. Along with urban size increase in central Taiwan, more pronounced effects of meteorological parameters were shown in the sensitivity experiment with urban cases. For the urban case (case SA, with urban size 15×15 km2), sensible heat flux is nearly 500 W m-2 around noon-time. It is about a factor of 3 more than the non-urban case (case CT) over the western plain. The surface air temperature in case SA increased by nearly 3 °C and humidity decreased with nearly 10% more than case CT. Along with the urban size increases in central Taiwan, the effects are the surrounding areas and were more pronounced not only the precipitation over downwind areas. Apparently, the unique and complex topographic features in Taiwan, including the lifting effect of the mountain bordering the western plain, are also play important roles for precipitation formation. Numerical study suggests that the heat-island effect over the western plain could perturb thermal and dynamic processes and hence affect the location of thunderstorms and precipitation over Taiwan's western plain.