Within the framework of the direct moment method, a theory for calculating the optical transmission of nanometric circular aperture on top of a metal coated optical fiber tip is developed. The primary purpose of the study is to estimate transmission efficiency of near field probes widely used in near field scanning optical microscopy. Field distributions immediately after the aperture in the core and cladding of the fiber tip can be accurately calculated in three dimensions. It turns out that the. transmission is a function of many variables, such as the radii of the aperture and fiber core, the thickness of the metallic coat, the material responses in the core and cladding area of the fiber, and finally the light frequency.