The effects of the dispersion mismatch between the reference arm and the sample arm of spectral domain optical coherence tomography (SDOCT) system on the spectral energy distribution in the spectrum of the image obtained with SDOCT are discussed. A method of numerical dispersion compensation in SDOCT is proposed, which is based on the concentration of energy in the image spectrum. It has been shown that when the percentage of energy in the low- frequency area of the spectrum is minimized, the dispersion mismatch is balanced. The plane mirror is taken as the sample in experiment to demonstrate the method, showing that the dispersion mismatch balance achieved with our method is in accordance with that by the full width at half maximum (FWHM) based method. Finally the optical cross- sectional image of a finger is obtained in vivo and the dispersion mismatch is balanced with the method. The results show that the method can effectively compensate the dispersion mismatch and improve the resolution of the images.