This paper presents a novel directionally adaptive cubic-spline interpolation method which is applicable to mobile camera digital zoom systems. The problems of conventional (linear and cubic-spline) and advanced interpolation exhibit blurring and jagging artifacts in the digitally zoomed image. To solve this problem, the proposed method performs directionally adaptive interpolation using the optimal interpolation kernel according to the edge orientation. Experimental results show that the proposed method successfully enlarges images with reduced interpolation artifacts compared with both conventional and advanced interpolation methods. Objective evaluation reveals that the proposed method gives higher peak signal to noise ratio (PSNR) and structural similarity (SSIM) figures.