Abstract: Co-based alloy is of a special microstructure with excellent properties so as to be widely used in various industries. However, different defects on Co-based alloy overlay have been reported while welding or operate an equipment with Co-based alloy because of a poor weldability from this alloy, one of which is the cracks. The cracks have been proved to show a most negative impact on an equipment. This paper is developed to study the crack morphology and its causes. Thus some miscellaneous cases on Co-based alloy were specially analyzed and described, and the typical basic laws and usual methodology causes were raised and finally some suggestions to limit the cracks on Co-based alloy overlay were summarized.