This report presented the method for thermal neutron dosimetry on base of determination of specific radioactive activity of measured 24 Na in the irradiated Human blood. Stable sodium contents of 23 Na in blood samples (for 150 people in Lamdong province, in which there are 85 staffs working at the Nuclear Research Institute) determinated by the method of instrumental neutron activation analysis at the Reactor and have the average value of ± mg/ml blood.