Abstract: A heavy rain process occurred in South China on 23 May 1998 was a typical case during the intensified observation period of HUAMEX. Satellite images revealed th at two types of MCSs caused the precipitation, Type A was generated on the Mei- yu front, and Type B was generated in the prefrontal region. Using the PSU/NCAR MM5 model, a 36-hour numerical simulation of this case was conducted initiating at 00:00Z 23 May 1998. It was verified that the amount and area of precipitatio n, cloud distribution and the configurations of synoptic systems matched quite well with observations. The model also reproduced the process of cold front movi ng southward and MCSs occurring both on the front and in the warm area simultane ously.Features of the two types of MCSs' structure associated with Mei-yu front were investigated. Both of them have some common characteristics of MCS. There are me so low and high centers located to the bottom and top of MCS respectively. Corre spondingly, wind fields converged at the low level, while diverged at the top of MCS. Additionally, both of them have warm core structures. However, their difference was also distinct. Type A was triggered by the cold front, and the prevailing lifted southeastly flow was its main provider of water vapor and unstable en ergy. Type B was a deep convective system occurring in the warm area ahead of th e front, which was triggered by mesovortex. Consequently, Type B was suggested to be more baroclinic than type A. Using trajectory techniques, three dimension al flow structures of the MCSs were revealed. It shows considerable acceleration presence in the inflow in the low troposphere and the anticyclonic outflow in the top of both MCS, which proving the existence of the meso high and outward p ressure gradient force locating on the top of MCSs. And the air stream originati ng from the cold area was identified to penetrate the front and move upward into Type A.