One severe mesoscale convective system with damaging straightline wind occurred on 21 June 2005 in northern Shandong Province. Based on the analysis of lightning activities, radar echoes and cloud images, it was found that the cloudtoground (CG) flash rates increased rapidly at the initial developing stage, and they were very high (about above 20 fl per 5 minutes) all along the mature stage. At the dissipating stage, positive CG flashes gradually dominated. Almost all negative CG flashes occurred in the convective precipitation regions, whereas most positive CG flashes corresponded to stratiform precipitiation regions. The hourly flash rate lagged behind the minimum brightness temperature, and the area of cold cloud shield with temperature colder than -50 ℃ lagged behind the hourly flash rate. Strong wind at the surface occurred in or near the regions with dense CG flashes.The pulse of CG flash rate was slightly ahead of the occurrence of severe surface wind. The damaging straightline wind was due to downbursts. The maximum wind speed observed at weather station was approximate to the maximum potential speed calculated by WINDEX.