This research work presents a perfect method to determine yarn diameter based on scientific understanding of things nature and avoiding the obstacles of classical methods of yarn diameter measuring by magnification or calculating it from empirical formulae. The suggested method presented here depends on putting yarn in a geometrically well defined form and getting use of the relation between yarn length, yarn diameter, and yarn weight through this selected form. Yarn length, in this state, is a common factor between the selected geometrical form and the weight of the formed yarn. The relation between yarn weight and yarn length is well known through the count. As count is unknown, yarn is formed with changing length of its pattern, By this arrangement, two sets of informations are obtained. Yarn count, being a common factor between the two sets, can be cancelled. This results in a simple relation from which yarn diameter can be determined to the nearest 0.1 micron or more accurate. Details of forming device called EYDM (Estimated Yarn Diameter Meter) and testing procedure are indicated