Fluid flow through fractures in rocks depends upon the roughness of the fracture surfaces. In this work the height of both surfaces of a fracture in a core from the Higashi Hachimantai geothermal test field in northern Japan have been measured and analyzed to determine the monofractal behavior of the surfaces and resulting aperture, and the extent of surface matching. We have used these parameters to create synthetic fractures that are extremely good imitations of the measured fracture by using a new technique for controlling surface mismatching. These numerically synthesized fractures have been used in flow modeling by solving Reynolds equation in the three dimensions, and for calculating the transmissivity of the field fracture. The modeled transmissivity showed good agreement with the fracture transmissivity obtained from field pressure tests, and indicates that such an approach is able to estimate larger scale flow through real sub-surface fractures.¿ 1997 American Geophysical Union