This paper presents an analysis on the air flows in a single stage centrifugal fan with a vaneless diffuser and peripheral louver outlets. The performance data of the fan are obtained experimentally. A numerical fluid flow model is developed. The agreements between the numerical and experimental results are reasonably good. The performances of the fan under various working conditions are explained by the flow structures and pressure distributions. Flow energy analysis shows that the major energy losses in the fan are the impeller loss, the diffuser loss, and the gap leakage loss. Suggestions on the design and application of fans with such architecture are given.