Abstract: The tropical cyclone is one of the most destructive natural phenomena on the Earth. The mechanisms for the tropical cyclone genesis remain enigmatic and controversial， because of the lack of observations over the tropical oceans and the complexities of tropical cyclone lifecycle. Some observations and numerical studies indicate that the mesoscale vortex merger is a possible triggering mechanism for the tropical cyclone genesis. So far, however, there has been no realcase numerical simulating study on the vortex merger in the monsoon trough over the South China Sea. In this study, a high resolution simulation (4 km) is conducted to investigate the vortex merger process during the formation of the tropical cyclone Durian (2001) over the South China Sea by using the Weather Research and Forecasting (WRF) Model. The model verification shows that the formation and the development of Durian were reasonably reproduced. Consequently, the characteristic of the vortex merger was analyzed. Then the time series of vorticity budget were investigated. Moreover, two sensitivity simulations were performed to examine what role the merger process played in Durian’s formation. The results reveal that the midlevel vortex V2 which formed after 30 h integral is the main vortex for Durian’s genesis and that the lowlevel vortex V1 which preexisted at Philippines makes just an assistant contribution to its genesis. During their merger, the vorticity divergence first intensified at low level, and then the vertical advection transported positive vorticity to the midlevel. Their final merger accelerates vertically the establishment of positive vorticity, leading to Typhoon Durian’s formation.