The Sirocco fan performance and its three-dimensional flow characteristics are numerically prediction by STAR-CD. Turbulent flow computations are performed using approximately 500,000 mesh points, and the performance results of tow computational methods, transient and quasi-static flow analyses are compared with experimental data. In the present study, our attention is focused on the three-dimensional flow characteristics of the Sirocco fan blades and the secondary flow structure in the scroll. For a design optimization study, the scroll shape is titled by $10^circ$ to modify the secondary flow structure, which yields some improvement of the fan performance.