There are two characteristics of the behaviors of salt water in PRO membrane module, macro flow in the membrane module and nano behavior in vicinity of the membrane surface. It is proposed to use the numerical simulations in macro and nano fluid behaviors. It is focused on the advection of the salt water in the hollow fiber membrane module and the diffusion of molecules in vicinity of the membrane surface. The macro flow and nano behavior of salt water are simulated with 3-dimensional advection diffusion equation and the molecular dynamics, respectively. It is pointed out for the macro behavior that the distorted flow exists in the membrane module, therefore the concentration distribution is not uniform in the module. So the improvement of the membrane does not always improve the module performance. The membrane performance is closely depended on the flow condition of salt water at high performance membrane. The nano simulations indicate that the interaction between salt water and membrane surfaces affects the diffusion coefficients of the water in the vicinity of surfaces. There is a strong correlation between the diffusion coefficients of water in the vicinity of the surface and the contact angles of water on the surface.