An automotive sirocco fan has been widely used in air-conditioning devices. It is essential to understand the flow characteristics of an automotive sirocco fan to satisfy the trends for more efficient and less noise. In this study, numerical calculations were conducted to explain the three-dimensional unsteady, incompressible flow characteristics and performance of the sirocco fan using FLUENT. Reynolds-averaged Navier-Stokes equations with the standard $k-{varepsilon}$ turbulence model were used for the numerical analysis. The sliding mesh was used to simulate the relative transient motion of impeller against scroll. It was found that the inactive zone and the secondary flow reduce efficiency because they cause internal losses. A parametric study was performed to improve efficiency of the sirocco fan, using two geometric variables. Reducing the width of impeller could decrease the relative size of inactive zone and secondary flow. Changing the position of cut-off had little influence on the efficiency. Thus an improvement of efficiency could be achieved by reducing the width of impeller.