The influencing systems and the developing process of a hail event occurred on 30 May 2005 in Gansu are analyzed with cloud and radar echo images in detail in combination with the actual weather situation, circulation pattern, and environmental fields. The characteristics of hail cloud echoes, velocity echoes and satellite cloud images are obtained. The findings conclude that the event w as caused by a mesoβscale convective system rather than a zonal hail cloud system. Some quantitative indexes are also achieved, which is meaningful to the operational hail forecast.