A moderate earthquake （Ms5.0） occurred on 30 March 2008 in Huangcheng town, Sunan county, Gansu province. The principal parameters and sequence character of the event are summarized, the earthquake focal mechanism is inversed with P motion, and distribution of the seismic damage is ana- lyzed based on the date of the field investigation. The tectonic background of the event is discussed in the views of regional tectonics, seismic activities, focal mechanism, distribution of aftershocks and the major axis of epicenter area, but the result shows that the causative structure is not clear.