Abstract: On 21 July 1998 a flash flood associated with a meso-β convective system occurred in Wuhan, producing 88 mm rainfall in an hour. A diagnostic study of the genesis and development of the mesoscale convective system (MCS) is presented. The conclusions are as follows: (1) The flash flood occurred during the rapidly developing stage of the MCS. (2) There was aboundent moisture gathered in the lower troposphere and the atmosphere was nearly saturated with a big store of convective available potential energy (CAPE). (3) The MCS became nearly stationary in the vicinity of Wuhan after formed and a great amount of rainfall was accumulated in this area.