Abstract: Carbonides were discovered in HQ-80 steel coarse grain boundary after reheating by scanning electron micrograph. Energy spectrum analysis tested high intensities of Cr,Mo in carbonides. Implant test and Charpy-V impact test for coarse grain zone were conducted. Mechanism of HQ-80 reheat crack is due to low grain boundary strength because of poor Cr and Mo layer by separating out carbonides in boundary. Three basic influence factors on reheat crack are Cr,Mo,C elements reacting out carbonides, overheat and reheat cycles, restrain stress. Mechanism of reheat embrittlement and reheat crack is the same. Reheat embrittlement cannot be avoided by reducing stress. The implant test was successfully achieved by non-notch implant. The implant curve fracture verified that major factor on reheat crack is sensitive coarse grain in heat affected zone.