Recently the noise reduction of a small axial flow fan for cooling electronic devices is required. When the obstacles such as a fan guard grill and motor stays are located in the back of a fan, the 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 generation mechanism of the fan with obstacle located in the back of the fan. GG model is the model with a fan guard grill. Free model is the model without a fan guard grill. The flow pattern around the fan blades and pressure fluctuation on the fan blade surfaces were compared under the conditions of with and without obstacle. The fan noise in the GG model is higher 1 dB(A)than the noise in the free model. The pressure fluctuation at pressure side surface of blades in the GG model becomes high compared with free model in the noise with frequency component of 133Hz caused by the number of motor stays. The existence of a fan guard grill and motor stays caused the reduction of axial flow velocity at the blade tip, which increased pressure fluctuation on pressure side blade surface due to the interference between tip vortex and blade surface.