Abstract The dam is considered a huge structure that has multiple purposes in the life of a human. The most important role of a dam is the production of electricity, and the storage of a huge quantity of water. Therefore, it is very important to be vigilant in the studies made for it. Lebanon has indeed been designated as a seismicity country. However, the number of dams is increasing due to several natural consideration. Mseilha dam is an executed medium arch dam, which faced several problems during and after construction. The initial problem that faced a dam is the seismic force. This natural force is very dangerous for any structure, especially for a dam. It could increase the existing concrete stresses. These stresses will cause the development of cracks all along the dam. Once the concrete shows cracks, its resistance will decrease, and it will become more vulnerable to external effects. Several studies were made to fortify it and make it resistant to seismic force. To show the major effect of this force, two methods ‘pseudo-static’ and ‘response spectrum’ were applied to a medium arch dam. Therefore, to evaluate the performance of these two methods, the dam is modelled using finite element software. The obtained results show the importance of using the appropriate model in the design of such dams without taking the appropriate model.