Investigation of a 3D flow behavior in a single fracture is important to evaluate productivities of fractured oil or geothermal reservoirs. The Reynolds equation is conventionally adopted for a simulation of fluid flow in a single fracture, but the vertical component of the flow cannot be estimated in this method. To overcome this problem, the Lattice Boltzmann Method (LBM), which is a new numerical simulation method for the fluid flow, is applied in this paper. Consequently, the 3D flow in a single fracture is exactly evaluated by using the LBM.