Abstract:This paper presents a diagnosis of an intense developing extratropical cyclone that occurred in conjunction with a 500 hPa trough merger over the Bohai and Yellow Sea in March 2007. To analyze the dynamics of the cyclone development, the NCEP analysis data and the Zwack Okossi （Z O） vorticity tendency equation are employed. Results show that there are three phases during the cyclone development. Firstly the cyclone appeared at the lower troposphere with significant frontogenesis and baroclinity. Secondly the approaching of the 500 hPa trough increased the vorticity advection above the cyclone, and the difference between the upper and lower levels accelerated the cyclone’s development after the cyclone entered the Yellow Sea. Finally, the 500 hPa trough merger increased the cyclonic vorticity in the mid level troposphere which in turn enhanced the ascent movement at the mid and lower levels, and increased the latent heat release. Thus its central pressure reached the lowest.