In this article, the performance improvement of an in-line centrifugal fan obtained by utilizing outlet guide vanes is presented. For this purpose, numerical solutions were sought for Reynolds-averaged Navier-Stokes equations using a commercial finite-volume-based solver assuming three-dimensional, steady, viscous, and incompressible flow. Computational modeling was first verified by experiments that comply with the fan performance testing International Organization for Standardization Standard BS EN ISO 5801. Streamlines at the outlet revealed the necessity of guide vanes to eliminate the high circumferential velocity component and to recover static pressure rise. For this purpose, manufacturers employ flat plates welded on the fan chassis that carry the motor and the impeller. In the experiments, maximum fan efficiency was 17% without any flow straightener, whereas the flat plates increased the efficiency to 31%. Using the guide vanes designed in this study, the swirl at the outlet was almost completely eliminated, and the maximum efficiency was found to be 38%.