One of the main components of industrial complexes is steel ground tanks which their performance during and after an earthquake event is of great importance. In recent years, innovative low-damage technologies have been proposed by researchers to mitigate nonlinear damage in structural elements and also to decrease time and cost of repairs needed for post-earthquake serviceability of structures. One of these technologies is rocking mechanism which, as a method for limiting seismic demands, could be employed for seismic behavior improvement of tanks. In this paper, a cylindrical steel ground tank with low aspect ratio (D/H less than 0.75), was analyzed under horizontal earthquake excitation. The seismic response of tank in two different configurations; a) anchored or fixed to the base and b) unanchored with free rocking motion over base were calculated and compared. Fluid-structure interaction was considered by using lumped mass analogy and the OpenSees software was employed for modeling and analysis. Furthermore, the bottom plate of tanks was considered rigid and restrained against sliding. The calculated responses include response of fluid-structure interaction system, roof displacement, base shear force and overturning moment. The results demonstrated that a) base shear forces and overturning moments in unanchored tank is reduced by 9% to 26%, b) displacement response of tank roof and fluid impulsive mass is increased in unanchored tank, and c) the increase in displacement response of fluid convective mass in unanchored tank is less than 3% of that in anchored tank.