In this paper we show how resolution enhancement and autofocusing in digital holographic microscopy (DHM) is
obtained by using structured illumination. The structured illumination is generated by a spatial light modulator, and
enables to project fringes of different orientations and phase shift without mechanical movement. The combination of the structured illumination and DHM improves the resolution of phase and amplitude imaging. Furthermore, the image plane is numerically determined by searching for the minimal deviation between the reconstructed images carried by different diffraction orders of the structured illuminations. The method can be applied to both amplitude and phase object.