Abstract: Though many cloud merging processes begin from the middle and lower parts of clouds, but the authors find that many merging processes begin from the middle and upper parts of clouds. Because of the important effect of cloud merger, the authors studied a merging process on 17 May 2005 using a cloud-resolving mesoscale model WRF (Weather Research and Forecasting Model). In this process, many mergers happened. Observation and simulation results show the process goes through three stages: (1) convective cells merging into a convective cloud; (2) convective clouds merging into a convective cloud cluster; (3) cloud clusters merging into a cloud system. The first and second stages of the merger process begin from the middle and lower parts, often between the young or developing clouds. The third stage of merger begins from the middle and upper parts, often between the mature cloud clusters. These two kinds of merging mechanisms are quite different, but both are related to wind shear and updraft velocity. In this paper, the authors set forth these two kinds of mechanisms in detail.