The collimation of monochromatic light in optical metrology is of great importance for its determination of the accuracy of physical measurement. This paper proposes a novel collimation testing method with the differential grating (DG) characterized by its high sensitivity. In the process of measurement, the monochromatic light is incident upon a linear grating and is then projected onto a DG to create two sets of linear moire fringes. By analyzing the relationship between the phase of moire fringes and the divergence or convergence angle, the collimation of monochromatic light can be easily measured. Compared with the traditional collimation methods, this method is more accurate with an experimental accuracy of 10(-7) rad, which is in good agreement with the theoretical prediction.