Abstract: From the evening of 3 July to early morning of 4 July 2006,a mesoscale convective complex (MCC) from northern Jiangsu to the Yellow Sea spawned a series of tornadoes,linear convective winds and heavy precipitation. In this paper,the synoptic weather background and the MCC structure are analyzed in detail using conventional surface and upper-level observations,regional automatic weather station observations,satellite cloud images and Doppler weather radar data. The main conclusions are as follows. (1) In the upper levels,strong divergence developed in response to the background field. In the middle levels,the MCC was located in front of an eastward-moving short-wave trough to the northwest of the subtropical high at 500 hPa. In the surface,the MCC was located in the warm zone of a frontal cyclone,in the presence of medium-to-strong convective available potential energy (CAPE) and large vertical wind shear in a deep layer (0-6 km) as well as in the low level (0-1 km). (2) The main vertical circulation exhibits some characteristics. The southeasterly flow in lowest layer and the warm,moist southwesterly flow in the middle-lower levels transported air mass from the south of the MCC to the center of the MCC. Northwesterly flow prevailed in the rear of the MCC,was relatively dry entrained into the MCC,causing evaporative cooling and strong descending motion. The downdraft with momentum from northwesterly winds builded a meso-β scale cold pool near surface,and the meso-β scale gust front formed along the boundary between the cold pool and the surrounding warm and moist airmass. Cold high pressure formed from the ground to the lower troposphere (thunderstorm high pressure),while the meso-β scale low pressure developed in front of the gust front. A warm core emerged in the middle and upper levels of the MCC due to the latent heat release caused by condensation. In the middle troposphere,the MCC was characterized by a meso-β scale cyclonic vortex and low pressure; in the upper troposphere,meso-β scale anticyclonic divergent flow prevailed. (3) Doppler weather radar observations revealed that the MCC at mature stage mainly presented a linear structure. The main component of the MCC was an active bow squall line on the scale of 150-200 km,while several weak convective rain belts with a cyclonic curvature existed. Several rain belts merged into the common vortex center,which coincided with the center of the frontal cyclone in the ground and also the center of the meso-β scale cyclone of the corresponding MCC with a diameter of 40-60 km. (4) Multiple mesovortices appeared in front of the active bow squall line,and four EF2 tornadoes and two EF1 tornadoes occurred in these mesovortices. The mesovortices had horizontal scale of 4-5 km,with the rotation speed close to strong mesocyclones in super-cells but less vertical extension,and the formation mechanism was also significantly different from cyclones in supercells. (5) The scale of the cloud system at the mature stage of the MCC was 1000 km in which the coldest part below 220 K (-52℃) was about 400 km. The main internal structure of the MCC included a 150-200 km long active bow squall line,a meso-β scale cold pool and the gust front in the ground,and mesoscale vortexes on the scale of 4-5 km in front of the bow squall line. Some mesoscale vortices led to the formation of EF1 and EF2 tornadoes on scales of only tens to hundreds of meters. The entire system showed obvious multi-scale structural features.