The combination and optimization of the cyclic modes in neutron activation analysis (NAA) at the Dalat research reactor were performed to determine short-lived nuclides. This work focused on determination of selenium in biological materials using 77mSe (T
1/2 = 17.4 s) as a case in point. The detection limit was significantly improved by a factor of 3–4 times in comparison with the conventional NAA. The precision was typically within the range of 4–6 % at the third or fourth cycle. It is a reliable technique for determination of selenium in biological materials at sub-ppm concentration levels.