This paper presents a model for effect of fringing fields on the radius of Circular Micro Strip Antenna (CMSA). The model is straight forward, simple and accurate for determining the physical radius of the circular patch. It is proposed that the extension in physical radius of CMSA is directly proportional to the physical radius itself and also to the normalized thickness of the dielectric substrate. The new model gives good results without any iteration. Novel results are that for all CMSA(1) the ratio of extension in physical radius to physical radius is equal to H(2) ratio of fringing field area to physical area of the patch is 2H(3) ratio of extension in physical radius to the height of the substrate is a constant Anm/2π. Here H is the normalized substrate thickness. Normalization is done with respect to guide wavelength. Anm is the mth zero of the derivative of the Bessel function of the order n. Large number of CMSA have been designed and simulated to validate the new thinking. Some typical data and simulation results have been incorporated in this paper.