Design study of gas cooled fast reactors with natural uranium as fuel cycle input has been performed. The reactors utilizes UN-PUN as fuel, helium as coolant, and can be operated without refueling for 10 years in each batch. Reactor design optimization is performed to utilize natural uranium as fuel cycle input. This reactor subdivided into 10 regions with equal volume in radial directions. The natural uranium is initially put in region 1, and after one cycle of 10 years of burnup it is shifted to region 2 and the region 1 is filled by fresh natural uranium fuel. This concept is basically applied to all regions. To achieve criticality requirement relatively high fuel volume fraction is applied. In this study several parametric survey were performed for several parameters such as fuel-to-coolant volume fraction ratio, core radius, and core height. After some optimization process we determine a standard core with a height and a diameter of 350 cm and 240 cm respectively, and the volume fraction for this design is 65% fuel, 10% cladding and 25% coolant. Calculation has been done by using SRAC-Citation system code and JENDL-3.2 library.