Axial superresolution in optical coherence tomography (OCT) by a three-zone annular phase filter is demonstrated. In the proposed probe of a spectral domain OCT system, the width of the central lobe of the axial intensity point spread function is apodized by the filter to be within the coherence gate determined by the light source, while its sidelobes are lying outside the coherence gate without contributing to the coherence imaging. By measurement of the depth response of the OCT system before and after inserting the filter, an improvement of about 20% in axial resolution is confirmed. OCT imaging on biological sample of orange fresh is also conducted, demonstrating increased depth discrimination without the negative contribution from sidelobes realized by the phase filter in combination with the coherence gate intrinsic to OCT. It comes to a conclusion that we can obtain axial superresolution by filter in OCT system without the degrading influence of large sidelobes.