In the study of digital holography, an in-line optical arrangement is often made to record holograms so as to exploit the finite space band-width product of CCD sensor sufficiently. However, when an in-line hologram is used to reconstruct original object wave, its conjugate wave will coexist in the diffracted field. Separating desired component from conjugate component is very difficult. The effective elimination of conjugate image during numerical reconstruction through in-line hologram has been a crucial technique, which facilitates and promotes the application of digital holography. Based on Fresnel diffraction calculation and its inverse calculation, an iterative algorithm to reconstruct object wave from in-line digital hologram is presented. According to this method, conjugate component can be removed from reconstruction to get original object wave by utilizing the difference of their diffractive characteristics. The object wave can be reconstructed numerically through single in-line hologram. The computer simulation demonstrates that the method proposed here is feasible and easy to operate.