Abstract: Tow major mesoscale convective clusters of different characters occurred in the heavy rainfall event in Guangxi and Guangdong Province on 20 June 2005, and they are preliminarily identified as a frontal mesoscale convective system (MCS1; a frontal cloud cluster) and a non-frontal MCS (MCS2; warm sector cloud cluster). Comparative analyses on their convection intensity, maintenance mechanism, and moist potential vorticity ( VMP) structure were performed. The MCS1 in Guangxi had a frontal structure, and there were strong northerly inflows at the middle and upper levels of the system. The convection intensity analysis suggests that the ascending motion in both the frontal MCS1 and warm sector MCS2 were strong, it is hard to conclude whether the intensity of the frontal convective cluster was stronger than that of the non-frontal convective cluster, and their difference in precipitation might result from differences in their moisture conditions. The comparative analysis of the maintenance mechanisms of matured MCS1 and MCS2 show that in the MCS1 there were northerly inflows in the middle and upper levels, the convection was mainly maintained through convective-symmetric instability mechanism; while in the MCS2, the water vapor was abundant, the convection was maintained by moist convective instability mechanism. The structural analysis of V MP indicates that (1) the V MP patterns imply that the two clusters were both potential symmetric unstable in the middle and low levels; (2) the distributions of V MP1 indicate that there were the interactions between the cold/dry air and the warm/wet air in the frontal MCS1, and the interactions between the upper level jet and the low level jet in the warm sector MCS2; (3) the different structures of V MP2 demonstrate that high-level jets, low-level jets and moisture nearby the convective clusters had different impacts on the two types of convective systems, respectively.