In this research, the occurrence of hail analyzed based on instability indices in Northwest of Iran. For this purpose, meteorological codes related to hail (27, 87, 89, 96 and 99) in the years of 1995 to 2018 were extracted and analyzed statistically. Instability indices such as CAPE, LI, KI, TTI, L.C.L, PW and SWEAT were used for hail patterning. For the days of hail occurrence, instability indices were calculated from the skew-t diagram of Tabriz station. Then, hail occurrence days were clustered by using hierarchical cluster analysis method. After that, a representative day from the third cluster was selected and examined synoptically. The cluster results showed that hail occurrences were classified into three pattern. In the first pattern, the instability indices were weak and hail has occurred at the synoptic scale. The second pattern showed the instability conditions in the atmosphere with the occurrence of hail and the ascent of air in the region. In the third model, the instability indices showed the intensity of the thunderstorm and the occurrence of hail. In this model, the Convective Available Potential Energy (CAPE) has increased, which indicates the buoyancy and ascent of the air. In the third pattern representative day, the formation of cut-off low at 500 hectopascal level and the presence of low pressure in the Southeast of Iran has caused hail in Northwest of Iran.