Approximating fundamental mode of graded-index fibers by Gaussian-modified Bessel function, the power coupling between two neighboring graded-index single-mode fibers is calculated. Coupling coefficient formula is derived,curves of the coupling coefficients as a function of distance D with different F values,exponent g of power law-index are plotted. Comparing these results for step-index (as a limit of g = ∞) with Snyder s results which suit to step-index only, we see that Gaussian-modified Bessel approximation is better than Gaussian approximation in dealing with evanesent coupling problems.