Abstract: The initiation and organization of mesoscle convective systems (MCSs) that produced heavy rainfall over Yangtze-Huaihe basin during 26-28 June 2015 are investigated based on diagnostic analysis of observations and NCEP FNL analysis product. The results show that two linear MCSs that developed to the south of the Meiyu front directly contributed to the heavy rainfall formation. There existed two types of organizational modes during the development of the MCSs. Repeated back-building and eastward echo training of convective cells were two major developing modes of the MCSs from the early night on 26 June to early morning on 27 June 2015. From the morning on 26 June till the daytime on 27 June, new cells continuously initiated in the southern end of the MCSs and formed several NE-SW oriented rainbands. Two scales of convective elements were found during the mature stage of the MCSs, i.e. the northeastward "echo training" of convective cells along individual rainbands and the eastward "band training" of the NE-SW oriented rainbands along the quasi-linear MCS. Continuous back-building and echo training of convective cells along the same path and "band training" of the NE-SW rainbands contributed to the development of the MCSs. Orographic lifting of the unstable air ahead of the Meiyu front and the boundary layer convergence were the major initiation mechanisms for convection. Cold pools played an important role for the initiation, organization and maintenance of the MCS. Note that the characteristics of organization and evolution of MCS were restricted by environmental background in the surface.