Abstract: K0-values of 15 nuclides used in reactor neutron activation analysis were measured with a degree of reproducibility (2-10%). Measurements were carried out at different irradiation conditions (8<Φth/Φe< 117) using 97Ru and 103Ru as flux ratio monitor and comparator nuclides at the heavy water reactor of Institute of Atomic Energy. In the mea surements of K0-values, the non-1/.E epithermal spectrum, the burn-up of target, the true concidence effects of γ-rays in cascade and the measurement geometry were considered and the consequent errors corrected.