For the optimal design of an automotive blower system, effects of the scroll cut-off starting angle, the clearance between blade tip and bellmouth, and the scroll expansion angle on the performance of sirocco fan are investigated experimentally. Best performance is achieved at fan exposure ratio $Deltae/r_c$, =1.0, and clearance ratio $DeltaeC/C=0.62. Flow characteristics inside sirocco fan are also studied by using LDV. Flow patterns in the inside of fan can be classified into three regions. Velocity vector has the same direction as rotational direction of fan at 0~$120^{circ}$, toward the fan blades at 150～$180^{circ}$, and opposite direction at 210~$330^{circ}$. Turbulent intensity is relatively high near the cut-off edge in the scroll housing.