Conventional X-ray radiography is based on absorption contrast and assumes geome trical optics. An “in-line” method and theory of hard X-ray phase-contrast ima ging were introduced. The differences between the conventional X-ray absorption contrast image and the phase contrast image were distinguished by using radiogra phs of biological samples. The effect of X-ray source size on the image contrast was investigated. Moreover, the key parameters of absorption contrast image and phase contrast image were simulated theoretically and discussed.