Abstract: A nonlinear least-squares method was applied to determine the photoelastic fringe constant from isochromatic phase field obtained by digital phase-shifting photoelasticity. To perform automatic evaluation, the photoelastic fringe constant and the center coordinates of a disk under diametral compression are determinated simultaneously using the nonlinear least-squares method. A set of over-determinated nonlinear equations are obtained to estimate the unknown parameters by an iterative procedure based on Newton-Raphson method. Experimental result shows that the parameters are evaluated accurately from the isochromatic phase field obtained from the six-step phase-shifting photoelasticity. The photoelastic fringe constant and the disc center coordinates can be evaluated easily, simply and automatically by the technique.