The safety of large man-made structures, the environmental conservation and the expansion of associated mitigating measures in the case of natural disasters, require a good understanding of the causes and the mechanism of the structural behavior process. Given that the environment of the soil and rock, are formed as heterogeneous from three-phase solid, liquid and gas in determining the actual characteristics and performance in different circumstances, there are many difficulties and ambiguities. Thus several tools for measurement of deformation and stress values in the structure seems quite necessary to record the behavior of geotechnical structures in various stages of construction and operation. Instrumentation is defined as a device that is installed in structure to measure a particular parameter. Using instrumentation data, earth dams are monitored and their safety is evaluated. Monitoring is Performance survey of a structure during construction and operation using instrumentation results. Although the tools are unique guides to provide the behavior of the dam body, however, if the logical software analysis can not confirm instrumentation results, the reason is defect of instrumentation system. In this thesis Sattarkhan and Sabalan dams were modeled using finite element method by Plaxis2D software. The performance of instrumentation was evaluated by software analysis. Back analysis was used to analyze the behavior of the dams. The numerical modeling was performed to the highest level of dams with assuming 2D plane-strain conditions. Construction of dams was modeled as layering in accordance with the timing of during construction. Behavior of dams during impounding was analyzed by reservoir water level changes. Analysis of Sattarkhan dam settlements showed good agreement between the instrumentation data and the software analysis. The settlement gauges are functioning properly. In addition, the settlement process during the operation proves that the dam becomes more stable over the time. Checking of Sabalan dam settlement during construction and operation indicated that the settlement gauges have reasonable performance. Meanwhile the software analysis predicted that Sabalan dam during operation moves towards sustainability. Analysis of pore water pressure in Sabalan dam during construction showed that dam consolidation during construction was not completed and continues during the impounding. During impounding piezometeric level of upstream to middle section were most impressed reservoir water level changes. Points near the drainage system in downstream of the dam showed Piezometeric level decrease over time, that were demonstrator proper functioning of the drain. The total stress of the dam has an increasing trend during the construction and it was exposed to the changes of the reservoir water level during impounding. This effect was reduced from upstream to downstream. The arching of sabalan dam during construction was reasonable. The total stress and pore water pressure in some piezometers and cells pressure do not seem logical. The comparison of these instrumentation data with the software analysis indicated some differences, so they need to be checked. Key Words earth dam, instrumentation, monitoring, finite element, Plaxis2D, analysis