In order to describe the complexity of microcosmic cracks of rock,a new method is introduced to calculate the box dimension,which includes four steps: scanning by the electron microscope,processing of the scanned picture,covering with meshes,calculating by program.This method predigests the box dimension calculation of microcosmic cracks of rock,,improves the efficiency and credibility of the calculation and realizes the automatization of the calculation.It is very important to the engineering application.