A simple approach to deal with an engineering problem of free vibration of functionally graded beams with nonuniform cross-sections where the material properties arbitrarily vary along the axial direction is presented. Instead of directly solving the fourth-order governing differential equation with variable coefficients, we transform the governing equation to a system of linear algebraic equations based on the polynomial expansion and integral technique. Then, a characteristic equation in natural frequencies will be obtained. Several examples of estimating natural frequencies for axially graded beams and non-uniform beams are presented, which show that our method has a fast convergence and the obtained numerical results are accurate. The proposed method is important to investigate because of the dynamic behavior of axially non-uniform beams in engineering applications.