Instrumentation of dams very significant in all large dams by installing instruments as pore Water pressures, earth pressures and displacements ,during construction, first impounding and operation are measured and the performance of dams are evaluated and analyzed . In this paper the performance of Gavooshan dam in the west of Iran during the construction is evaluated with the help of the results of instrumentations. The performance of the dam in the future is also predicted using the result of numerical analysis. During this research it is observed that the performance of dam during the construction in terms of the magnitude of pore water pressures, the coefficient of pore water pressure “r_u”, displacement, stresses and the amount of developed arching has been appropriate in comparison with the performance of similar dams in Iran and in the world, and it is in safe condition. Also in back analysis, better parameters for core, transition and shell materials is obtained. Using these parameters the performance of the dam at the end of construction predicted.