Based on the NCEP-CFSR(National Centers for Environmental Prediction Climate Forecast System Reanalysis) reanalysis dataset and WRF model,this paper studies the formation process of initial vortex of tropical cyclone Nargis over the Bay of Bengal(BOB) in April 2008.Results indicate that the initial disturbance of Nargis,which is found in the northern Pulau Sumatera Island,is probably triggered by the equatorial westerly jet associated with an eastward MJO event over Indian Ocean.On the other hand,the easterly jet(22—25 April),which originated from mid-latitudes,passed by South China Sea and entered into BOB,plays an important role to intensify the initial disturbance to be a tropical depression.When the cold easterly jet enters into the eastern BOB,the available potential energy of low level atmosphere converts into the total kinetic energy by increasing sea surface sensible heating,then the low level cyclonic circulation increases rapidly,and the initial disturbance moves northwestward and finally develops into a tropical depression.Through the sensitive experiments with WRF model,it has further proved that the initial disturbance of Nargis will not develop to be a tropical depression if the easterly jet is very weak.