Abstarct: Dynamic analysis of a concrete arch dam assuming that it is a monolithic structure show large tensile stresses in arch direction, especially near the crest. These high tensile stresses are usually far larger than tensile strength of the concrete leading to the formation of crack in the high tension zones of the dam body. On the other hand arch dams are not monolithic shell structures but constructed as cantilever monoliths separated by contraction joints. These joints can not develop the tensile stress predicted in a linear analysis in reality the joints can be expected to open and close and slide relative to each other during the earthquake, releasing arch stresses and redistributing internal forces. Cracking or joint opening and sliding changes the behavior of the dam. Thus, it is obvious that under earthquake conditions, the assumption of linear behavior of structure is not reliable and results of linear analysis are not accurate enough to evaluate the safty of the dam under earthquake loading. Dam – Reservoir – Foundation interaction, water compressibility and opening and closing of dam contraction joint are important active parameters that influence seismic response of these structures. During earthquake the above mentioned factors make dynamic analysis complex. To avoid these complexities, the analysis of arch dam is usually accompanied by simplified assumption. In this research it is tried to evaluate seismic behavior of arch dams and the impact of all these factors by analyzing an arch dam as a sample, then a comparison is made between the results and those models with simplified assumptions. The ANSYS program was selected to carry out analysis due to the fact that it includes convenient element library to model water compressibility, joint and discontinuities. Morrow point arch dam is selected as a sample to carry out analysis and investigate the results. TAFT earthquake and NAGHAN earthquake are used for the time history analysis.