Mesoscale simulation of a flood producing rainstorm of 21 June 2004 over Rangamati, Bangladesh has been carried out using the MM5 Model. The model performance was evaluated by examining the different predicted and model derived parameters. The MM5 model suggests that the rainstorm over Rangamati was resulted by the large scale weather circulation associated with active conditions of southwest summer monsoon over the head Bay of Bengal which generated favourable conditions for developing the severe mesoscale convections. The strong confluence of southwesterly low level flow transporting large amount of moisture (which vertically extended up to 350 hPa) from the Bay of Bengal towards southeast Bangladesh and its neighbourhoods appears to be one of the striking features. An elongated vorticity maximum of the order of 30-40×10-5s-1 was observed across the Rangamati. A well-defined area of low level convergence (-25×10-5 s-1 ) is associated with strong divergence (25×10-5s-1) in the high level (200 hPa) has provided favourable conditions for strong convection. The model simulated realistic pattern of rainfall over Rangamati as compared with TRMM observations. Considering the quantitative comparison of the domain average rainfall with TRMM observations, it is found that the model over predicts by 12% as compared with TRMM observed rainfall.