To study the performance variations of an axial fan after impeller trimming, numerical simulations are executed under the following conditions: trimming quantity of 5%, 10%, and 15% of blade height, with tip clearance changed and unchanged. The effects of trimming quantity and tip clearance on flow field and performance of the axial fan are investigated, and formulae are presented for total pressure rise and flow rate ratio versus impeller diameter ratio and for operating points before and after impeller trimming. The simulated results show that the performance curves after impeller trimming tend to decline. Near the design flow rate, the performance of the fan with unchanged tip clearance is aerodynamically superior to that with changed tip clearance; the former deteriorates at a large flow rate. The margin of parameters provided by design units exceeding the actual operation requires the fan to normally operate to the left of the rated load, therefore, axial fans with tip clearance unchanged after impeller trimming have been found to maintain better performance. The equations, denoting total pressure rise ratio and flow rate ratio versus impeller diameter ratio are available to determine the trimming quantity according to actual requirements of operating points after impeller trimming. (C) 2014 Elsevier Ltd. All rights reserved.