Cylindrical steel above ground tanks extensively used in oil refinery complexes and oil depots in Iran. Past earthquake experiences in Japan, U.S.A, and Turkey show that these tanks are very vulnerable to strong ground motions and the study of the vulnerability and strengthening of these tanks are very important. In this paper, 5 steel above ground tanks with different ratio of height to diameter (H/D) have been studied and different failure mechanisms including elephant foot buckling, diamond elastic buckling, failure of roof due to fluctuation of fluid, sliding, overturning, up lift and unsymmetrical settlement were assessed using nonlinear analyses. International documented references such as ASCE and API650 standards and static, modal, spectral (linear) and time history (nonlinear) analyses is used in this study. Results of the analytical study show that the minimum freeboard of tanks should be 13 percent of the tank height to avoid of roof damage during an earthquake. In addition, tanks with a height to diameter ratio greater than 1.0 are unstable with the problem of overturning. Other failure mechanisms mentioned above are not dominant in the case studied tanks.