Abstract: The change in austenite grain size of a steel with low sensitivity to weld crack was investigated via high-temperature quenching test. The correlation between reheating temperature, holding time, starting temperature for austenite grain coarsening and micro-hardness was discussed via metalloscope/TEM observation, hardness test and the equations of second phase particles' solubility product. The results showed that the austenite grain coarsening temperature of specimens is about 1200°C. When the reheating temperature is lower than 1200°C, a great number of fine second phase particles will hinder the austenite grains from coarsening, and they will be dissolved and growing abnormally when higher than 1200°C. It was found that the reasonable reheating temperature range is from 1150°C to 1200°C, within which micro-hardness of quenched structure is lower than HV330.