With more and more applications of glass in advanced fields of science, the demand for glass machining precision and efficiency has increased greatly. More and more attention is being paid to glass cutting by ordinary tool because precise glass parts with various shapes can be obtained at high efficiency and low cost. But, in this method, rounded cutting edge radius of tool has important influence on precision and integrity of surface. This paper presents the laws of glass crack initiation and propagation when a cylindric indenter is used for glass indention. Research results show that two cracks that are symmetrical to the normal plane of the indentation surface occur suddenly when the load increases to a critical value. Closing occurs in the two cracks during unloading. The interior angle between the two cracks and critical load increase with rounded edge radius of cylindric indenter linearly.