Unique model for the reservoir in its full complexity is impossible to establish. Accounting and quantification of uncertainty is becoming a considerable part of reservoir management. Important amount of uncertainty concerning a number of reservoir parameters can still late exist in the life of field, notwithstanding the long operations time of oilfields. The theoretical and practical understanding of these parameters can have an eminent influence on the prosperous development and exploitation of the reservoir. These parameters are the relative permeability, the degree of anisotropy, to appoint only some. In the interest of the petroleum company TOTAL, two reservoirs in Syria and Iran were looked closely and unreliable parameters were recognized. The simulation sectors were established and confirmed with the software Eclipse. Sensitivities studies were performed on these parameters in order to appraise them, to bring out their meaning and to determinate their impact, based on the current status of knowledge about these reservoirs. The goal is to perform reservoir simulations for different values of the uncertain parameters with the purpose of getting a qualitative idea of each parameter influence. A detailed examination and issues of these parameters are described in the thesis. Besides recommendations for decisions in the future concerning the reservoir management and further field development for those reservoirs or another analogous reservoirs are inferred.