Abstract: A set of concentric interference circles is formed for diffused monochromatic light illuminated a F-P etalon. Fractional orders at the centre of the interference fringes are determined by excess fraction method. The spec- trum of the light source consists of certain adjacent wavelengths,including one He line,one Ne line, and a few Hglines. Thus images of the resulting sets of interierence circles are obtained simultaneously by a digital camera at the focal plane of the imaging lens. The diameters and standard deviations are computed by the circular regression of coordinate data. The fractional orders are calculated by weighted regressions in a new quasi-linear model, and the expanded uncertainties are assessed corresponding to different wavelengths. Inducing three of the wavelengths to the process to solve the fraction orders by excess fraction method,the spacer d of the F-P etalon is calculated to be d± Ad=（3 096 977.97±1.76）nm.

Key words: excess fraction method, Weighted regression, Fabry-Perot etalon, uncertainty of measurement