Fan Filter Unit(FFU) is a device which supplies clean air from ceiling in a clean room. When a big sized FFU is installed, the velocity variation between center and side of the FFU can decrease products’ quality or productivity. Therefore, a guide vane is installed inside the device in order to increase the velocity uniformity. Since the vane reduces the flow rate for a given pumping power, an optimum design is required to increase velocity uniformity and minimize flow rate reduction at the same time. To find a guide vane to achieve these requirements, a series of numerical simulation has been conducted while changing the angle and length of the center and side guide vane. By changing the geometry of the side guide vane, the velocity uniformity increased by 3.7%, and the flow rate decreased by 1.5%. With the change in the geometry of center guide vane, the velocity uniformity increased by 2.9%, and the flow rate decreased by 0.7%.