The performance of in-line duct fan depends on the design parameters of impeller and guide vane such as sweep back angle of impeller hub, guide vane angle etc. In this study four kinds of impellers having different sweep back angles, $0^{circ}$, $17.5^{circ}$, $35^{circ}$, $52.5^{circ}$ with 8 guide vanes, and different guide vane angles, $15^{circ}$, $30^{circ}$, $45^{circ}$ were selected and their performance measured to investigate the effects of design parameters. The results show that both sweep back angle of impeller hub and the guide vane angle have large effect on the efficiency. Especially, it was found that the mixed flow impellers having sweep back angle between $17.5^{circ}$ and $35^{circ}$ gave good performances for in-line duct fan.