Fan filter unit is an air conditioning device for a clean room to provide positive indoor pressure by supplying air. A uniform velocity at the exit of the unit is required to have high productivity at the clean room, while higher flow rate is desired to save the energy. 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 guide vane. By changing the geometry, velocity uniformity has been enhanced up to 3.7%, whilst the flow rate decreases by 1.5%. The flow rate can be increased by 2.6%, with the 2.1% penalty of the uniformity.