In this article, we investigate the eigenvalues of nonsingular fractional second-order Sturm-Liouville problem. The fractional derivative in this paper is in the conformable fractional derivative sense. We implement the reproducing kernel Hilbert space method to approximate the eigenvalues. Convergence of the proposed method is discussed. The main properties of the Sturm-Liouville problem are investigated. Numerical results demonstrate the accuracy of the present algorithm. Comparisons with other methods are presented.