A casing fence is a newly proposed casing treatment to enhance the aerodynamic and aeroacoustic performances of the axial flow fan that exhausts air into the atmosphere for cooling and ventilation. In the present study, the fence is applied on the shroud near the trailing edge of an axial flow fan used in an outdoor unit of an air conditioner operating at the design condition. The Reynolds number of the fan is 547,000 based on the radius of the blade tip and the tip velocity. We simulate turbulent flows and acoustic fields in the fan using large eddy simulation (LES) coupled with Ffowcs-Williams Hawkings (FW-H) equations. From the numerical simulations with the casing fence, the efficiency of the fan is increased by about 9.5% and the propagating noise is reduced by 0.5dB (A) compared with those without the fence. The fence decreases backflows induced by the tip-leakage vortex near the casing, suppressing the circumferential development of the tip-leakage vortex from the previous blade to the next blade. Also, it reduces the double-leakage tip-clearance flow generated at the trailing edge of the blade tip. As a result, the blockage effect by the leakage flow is reduced and the efficiency is increased, and pressure fluctuations on blade surfaces due to the tip-leakage vortex are reduced, resulting in a reduction of the propagating noise.