Recently noise reduction of a small axial flow fan for cooling electronic devices is required. When obstacles such as motor stays are located in back of a fan, noise of an axial flow fan becomes higher. In this study, numerical simulations were conducted by using LES (Large Eddy Simulation) with DSM (Dynamic Smagorinsky Model) about an axial flow small fan in order to clarify the noise reduction mechanism at high flow rate. The flow pattern around the fan blades and pressure fluctuation on the fan blade surfaces were compared under the conditions of different flow rates. The fan noise at high flow rate (Q=0.030 m³/s) is lower 1.9dB(A)than the noise at design point flow rate (Q=0.025 m³/s). The pressure fluctuation at pressure side surface of blades at high flow rate becomes low compared with the pressure fluctuation at design point flow rate. The axial flow velocity at the blade tip increases at high flow rate, which decreased pressure fluctuation on pressure side blade surface due to the interference between tip vortex and blade surface, and decreased pressure fluctuation on suction side surface due to the boundary layer separation.