Drought is an insidious environmental phenomena. This phenomenon causes water shortage for some activity, group or environment. Drought Should compared With Climate Conditions and  Hydrologycal Moderate in Long-term. Hydrological drought by reducing water storage in lakes, underground water levels and reduced river flow is detected And often "to affect a large area. The most common definition of a hydrological drought by defining a threshold that, if the river is less than a hydrologic drought is considered to be in this study, the threshold fixed (Q70 and Q90) to evaluate the hydrologic drought in the Karaj dam Basin. The results show that 1377 year was one of the driest years in this area and in Karaj station drought in 1377 year 284 days in 255 days Gachsar station is continued. The results show a total lack of water in Gachsar station Q70 threshold equal to 68/11 million cubic meters, equivalent to 75.2 million cubic meters Kalavan stations as well as in Karaj station 44/42 million cubic. As well as the general extreme value (GEV) best fit the parameters of the drought. The results can be used in water resources management and planning necessary to prevent or mitigate the effects of drought in the area.