The summer monsoon rainband in East China and its northward seasonal leap in relation to conditional symmetric instability (CSI) during the period of 1981 to 2010 is diagnosed through computing and analyzing moist potential vorticity (MPV) flux,CSI, slantwise convective available potential energy (SCAPE), and a newly developed index of conditional MPV flux (CMF). The European Centre for Medium-Range Weather Forecasts reanalysis data and daily precipitation data collected at 741 stations in China are used. Monthly-averaged anomaly fields show the northward leap of heavy rainfall center from South China in April to June to the Yangtze-Huaihe region in May to July and to North China in July to August. The spatial pattern of more precipitation in North China and less precipitation to the South of the Yangtze-Huaihe during July to August is well represented by the rainfall anomaly field than by the mean field. The negative MPV flux is in phase with the northward leap of heavy rainfall from South China (April to June) to North China (July to August). CSI zones within the rainband center are mainly located between 925 and 600 hPa in April, May and September, whereas they vertically shift to 700-600 hPa from June to August. The CSI zones are also a good indicator of the northward leap of summer monsoon rainband and its intensification as well as its southward retreat and weakening. During the late spring and early summer when the monsoon starts, the thermal wind contribution to SCAPE is dominant over the contribution of CAPE in the magnitudes, whereas the opposite is true from July to August. Furthermore, the thermal wind contribution is in phase with the northward seasonal leap of heavy rainfall from South China in April to June to Yangtze-Huaihe in May to July and to North China in July to August, while CAPE has no such relationship with the heavy rainfall center. The CMF index can be an indicator for the regional summer rainfall anomaly in East China.