On 1200 UTC 7 June 1998, a mesoscale convective system (MCS) hit Taiwan, produced over 300 mm rain in 24 hours. Massive flood caused severe disaster over southern Taiwan. This MCS was initiated on the south side of the Mei-Yu front on 0000UTC 7 June and well developed in the Taiwan Strait while moving to the southeast. Numerical study was carried out for this event by using the Penn State / NCAR Mesoscale Model (MM5) model. The model captured the evolution of the MCS, including the shape of clouds and the 24-h accumulated precipitation on Taiwan. In the mature phase of the MCS, a mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were clearly identified in the numerical simulations. At 850 hPa, the mLLJ was located on the southwest of a mesoscale low and directed toward the MCS. At 300 hPa, a mesoscale high was simulated over the MCS. The mULJ was located on the east of this meso-high. Strong mesoscale convergence (-3.1 ×10-4 s-1) was induced on the left-front (exit) region of the mLLJ. The convergence area overlapped with a strong divergence (3.6 ×10-4 m s-1) in the rear (entrance) of the mULJ. The maximum upward motion in this area reached 146 cm s-1. The vertical coupling of the mLLJ and mULJ was crucial for the torrential rain produced in MCS. In the lower troposphere, warm, moist and potential unstable monsoon air injected into the MCS from the southwest by the mLLJ. It rose rapidly in the left-front of mLLJ, and it only took about three hours to reach to the upper troposphere. The upper air flew southeastward out off MCS along the mULJ. A slant vertical circulation existed in the MCS, 100 km south of the Mei-Yu front. This kind of meso-scale vertical circulation was confirmed by trajectory analysis. Moisture concentration and momentum budget were also diagnosed in this study. We found that the horizontal moisture advection was rather weak due to the weak moisture gradient in the vicinity of the MCS. The meso-scale convergence of moisture flux was mainly contributed by horizontal convergence of the wind field on the left-front region of the mesoscale low-level jet (mLLJ). The mLLJ played an important role in the concentration of the moisture in the MCS. Momentum budget was calculated in the exit / entrance regions of the mLLJ / mULJ. The pressure gradient force, associated with the low-level meso-low, was found to be important for accelerating wind in the entrance and the center of the mLLJ. Although horizontal and vertical momentum advection acted to decelerate the winds at the entrance of the mLLJ, the pressure gradient force and Coriolis force compensated these losses and helped to maintain the strength of the mLLJ. In the exit region, horizontal advection of momentum flux accelerated the mLLJ, but vertical divergence of momentum flux tended to reduce the speed of the mLLJ. The pressure gradient force and vertical divergence of momentum flux were both found to be important for maintaining the mULJ. Strong convective upward motion, which carried the horizontal momentum upward from the exit of mLLJ to the entrance of mULJ, was crucial in the vertical coupling of the mLLJ and mULJ.