Gotvand dam is a rock fill dam located in Khoozestan province in Iran on Karun River. It will be constructed on weak mudstone and layers of sandstone. These layers are intermittent of medium strength with uniaxial compressive strength of 15 and 25MPa respectively. Some regional factors as continuous unloading, caused by river flood washed off and horizontal tectonic loading have created a local anticline in the base of the dam. It is estimated that such deformation will last over the time. Therefore, in order to determine creep parameters assessing the time dependent behavior of the foundation rock during the life of the dam through numerical and analytical methods is inevitable. In this research, by conducting creep tests under constant loads the strain- time graphs were produced for the two rock types. Applying the Burger rheological model for both rocks, their creep parameters were determined. In order to validate the produced data numerical modeling was conducted. A comparison between the results of numerical modeling and laboratory tests showed that the difference between the two methods is less than 6%. Also the time required for both rocks to enter the tertiary creep was determined to be 40 days for sandstone under 17.72MPa and 35 days for mudstone under 7.8MPa of load.