Using BrdU labeling to detect the HPRT gene mutation frequencies(MFs)of a normal adult induced by γ rays of different doses and to analysis the relationship between the HPRT gene MFs and the radiation doses. Blood was drawn via the vein of a normal adult and was equally divided into five groups. The blood was exposed to γ rays with a dose range from 0.0Gy to 4.0Gy respectively. After having cultivated the whole blood, we got the HPRT gene MFs of the lymphocytes. With the increase of the doses,the MFs of the HPRT gene rose accordingly. The result also showed that there is a linear—square relationship between the doses of the γ ray and the MFs of the HPRT gene. According to the result, we may conclude that BrdU labeling method is a quick,simple and relatively sensitive method for detecting the HPRT gene mutations of somatic cells induced by external radiation exposure and HPRT gene mutation may be used as a radiobiological dosimeter.