We propose a numerical method for phase curvature compensation in digital holographic microscopy, in which the phase curvature is compensated for by subtracting a numerical phase mask from the distorted phase. The parameters of the phase mask are obtained based on phase gradient fitting and optimization, in which the initial mask parameters are obtained by fitting the phase gradient, and then more accurate mask parameters are determined using a spectrum energy search. The compensation can be executed in a hologram without extra devices or any prior knowledge of the setup and specimen. A computer simulation and experimental results demonstrated the feasibility of the proposed method.