In this study the influences of non-uniform saturation distribution on the analysis and interpretation of pressure and pressure derivative curves is examined. The unusual behavior of the pressure transient data due to non-uniform saturation distribution makes the analysis of such reservoirs complicated and leads to ambiguity in the interpretation. In order to interpret the behavior of pressure transient data, which are influenced by changes of fluid saturation, a theoretical model is developed. The model application is demonstrated using a simulation in the reservoir containing oil and water with the variety of fluids saturation in the initial condition. The theoretical model provides a basis for understanding conclusions derived by numerical results as well as new insight for interpretation of pressure transient behavior affects by non-uniform saturation distribution.