Abstract: A method was presented for measurement of effective delayed neutron fraction β_eff by coupling experiment with theoretical calculation. In the experiment the reactor critical status was defined, and the subcritical reactivity under subcritical status, by using β_eff as unit, was measured. With theoretical calculations, effective multiplication factor k_eff under critical status was calculated, and the deviation of the calculated results was evaluated. Then, k_eff under subcritical status was calculated and also revised with the deviation as same to critical status. So the reactivity under subcritical status was evaluated. Comparing the experimental result with the calculated result, β_eff was defined. Since the reactor status was defined by experiments, the calculated k_eff under the status was little dependent on the theoretical models which described the reactor status. The results show that the accuracy of the measured β_eff is higher than previous measurements.