Eastern China experienced a series of severe floods during the summer of 1998. These floods are shown to be consistent with the propagation and activity of the 30--60 day intraseasonal oscillation (ISO) over the western North Pacific (WNP), where the monsoon trough and the subtropical anti-cyclone appear as an anti-clockwise propagation with the enhanced and suppressed convective anomalies in a 30--60 day period. The Meiyu circulation pattern in the lower atmosphere successively dominates the WNP, which results in southern China and the Yangtze River valley suffering much more rainfall and floods than normal. There are remarkable signals of 30--60-day ISO in convective anomalies over the Bay of Bengal and east of Philippine Sea. During the cycle of the 30--60 day ISO, these convective anomalies move towards the South China Sea (SCS), where they affect the convective activity of the SCS summer monsoon, therefore maintaining the cycle of 30--60 day ISO over the WNP.