By using MM5, the transition of Tom (a tropical storm in 1996 in North western Pacific Ocean) from the tropical storm to the extratropical cyclone is s imulated. The result shows that: (1) when Tom entered midhigh latitude of baro c linic zone, it interacted with the 500hPa westerly trough. The coupling of the w arm and moist air of low level at the east of the storm and the positive vortici ty advection before the trough made it possible for the cyclone to develop. The cold advection after the trough and warm advection before the trough made Tom wi th cold feature in the west and warm feature in the east. And this thermodynamic structure transformed Tom into a baroclinic cyclone. (2) The cyclonic shear left to the upperlevel jet stream e ntrance and the anticyclonic shear right to the upperlevel jet stream exit w ere favorable for the upward movement right to Tom and downward movement left to Tom, which made Tom transform from a barotropical structure to a baroclinic one . (3) The invasion of the cold and dry air in 700hPa level into Tom's warm core made the storm axis tilt NWSE from lower to upper level and destructed Tom's  w arm core. (4) Because of the invasion of the cold and dry air in 700hPa level, t wo warm cores appeared. At the north of the northern warm core a significant war m front was born and a cold front seemed to appear near the northwest warm core. (5) The subtropical cyclone's reintensification was related to the downward t ransmission of high potential vortex from stratosphere.