Abstract: A severe hail process occurs in Tianjin on 25 June 2008, based on the data of intensive AWS observation, Doppler weather radar, FY-2C geostationary satellite and NCEP/NCAR reanalysis data, analysis and diagnosis studies are carried out to seek the characteristic of this rare storm system and its evolution. The results show that with the cold vortex over North China, the upper-layer cold and lower-layer warm potential instability stratification is formed. In the left front of the core of upper-level jet, non-geostrophic effect enhances the development of the storm system. Three meso-β convective clusters merge into a quasi-circular structure meso-α scale convective system and maintain for about 4 hours in Tianjin. The CAPE increases rapidly before thunderstorm while the CIN decreases gradually, thus the lower atmosphere becomes slightly unstable from stable. As the storm approaches, the trend of θ_sewind profile is arcuate, and the convective instability atmosphere in the middle-low levels remain.Water vapour transported through southwesterly flow from the southern part of Hebei forms a high humidity area, and Tianjin region is located in the area of wet tongue. Wind speed difference between 700 hPa level and near-surface reaches 20 m/s at 14:00, SHR (0—3 km) increases significantly, reaching moderate intensity (6.5 m · s^-1· km^-1). Super cell storms merged by a number of multi-cells have direct effects on the hail during the development of its mature stage. With the aid of Doppler radar, bow echo, weak echo region are found in the lower level, strong hanging echo in the middle-upper level and three-body scattering spike (TBSS) are observed. VIL jumps significantly from the lowest value to 65 kg· m^-2before the hail occurs in Tanggu, and the top height of the storm remains 14 km, when VIL density increases to 4.6 g· m^-3, exceeding the threshold of heavy hail. Analysis indicate that the dry intrusion of upper troposphere and cold air from the middle troposphere near the ground may also be the triggering factor in this process, besides the northwest-southeast convergence line located in Tanggu and the northeast of Hebei Province.