One of the necessary structures in industrial installation and water supply system are tanks. Because of different purposes tanks are built as on ground, elevated, buried or half-buried. Tanks are used for storage of different materials such as water, oil, nitrogen, high pressure gases and chemical materials. Mostly, tanks are in two shapes of rectangular and circular. For water storage usually concrete tanks are used. These tanks are utilized for various purposes such as storage, providing pressure and calming fluid flow. Damage in these tanks can cause financial and vital injuries. Also, the necessity of providing water for a region stricken by an earthquake represents the importance of investigating their seismic performance for assessing their safe behavior. Because of widespread use of tanks and their vulnerability in earthquakes, notable damages have been observed during previous earthquakes. Sometimes these damages cause a full loss of tank performance and the fluid stored in it and the other time it causes only repairable faults. In this research the dynamic behavior of concrete cylindrical tanks is investigated. Effective parameters in dynamic response of tanks such as wall rigidity, base fixity, vertical acceleration of ground motion and height to diameter ratio are evaluated. Also for studying vulnerability of these tanks, seismic fragility curves for three different tanks with height to diameter ratios of 0.75, 0.5 and 0.25 are developed. Moreover, the effect of fill percentage on seismic fragility curves are investigated with considering three different cases for one of the tanks. The tanks are designed based on ACI 350.3 and evaluated by dynamic analysis of with Abaqus. It is assumed that the tanks rest on a rigid foundation. One of the most important issues in designing concrete tanks is control of crack width for preventing leakage and corrosion of rebars. In addition, knowledge of water wave height due to sloshing of water is important for considering the free board. Seismic fragility curves for critical crack width, bending capacity of wall and water over flow from tanks wall are developed. One other important problem about dynamic behavior of circular concrete tanks that has less been investigated in previous researches is the simultaneous effect of vertical and horizontal components of earthquake acceleration. The results of numerical analysis show that increase of wall flexibility has a considerable effect on the response of fluid and especially tank wall. Also use of a hinged base connection instead of a fixed base connection significantly reduces the maximum bending moment in the tanks wall. It is observed that the effect of vertical acceleration of earthquake should be taken into account in designing tall tanks. The seismic fragility curves for critical crack width and fluid spillover in tanks represented that the height to diameter ratio and the fill percentage are major sources of uncertainty in seismic fragility curves of cylindrical concrete water tanks. Key Words Concrete cylindrical tank, Dynamic analysis, Flexural crack width, Water sloshing, Seismic fragility curve.