The Fresnel diffraction of the grating illuminated by partially coherent light is analyzed based on the propagation of the mutual intensity in theory of partially coherent light. The general formulas of the intensity in the Fresnel diffraction field are obtained. Then the characteristics of the field diffracted by the grating is discussed in detail. Experimentally, the diffraction intensity of the grating illuminated by the light source (LED) with different illuminant area is measured in the Fresnel region respectively. Thus the basic characteristics and general rules of the grating diffraction are proved, for instance, the smoothness process of the source on to the diffraction intensity, namely, Talbot imaging, and the condition of gaining exact Talbot imaging. The experiment results conform to the theoretic ones very well