Many literatures on cost effects of financial institutions adopted the Translog cost function form. But the Translog cost form is a restrictive specification by itself. Different size should distribute over two side of a standard U-shaped quadratic average cost (AC) curve. To use Translog form to estimate scale economies and cost effects most get not only similar but inevitable results. It is sure to affect the accuracy of studies. This paper evaluates Translog, Fourier, and Spline function to establish cost system in Taiwan. The results show that the AC curves of Fourier and Spline function are similar to M-shaped, those estimations approximate actual costs of financial institutions. Furthermore, the outcomes of scale economies estimated with Spline function are more stable than those with Fourier function.