Abstract: Effective delayed neutron fraction eff is an important parameter in dynamic behaviour of nuclear reactor. Besides, eff has the role of the conversion factor between a relative unit and an absolute unit in reactivity scale, which is important for examining work of macroparameter according to reactivity. An experiment was performed to measure theeff in a fast critical assembly using Nelson number method which is based on Rossi- method. Using lead for shielding, adopting the thinner 6Li glass scintillator and pulse amplitude discrimination, the influence of is decreased. The measurements were carried out for reactivity ranging from -60 ￠ to delayed critical state. The final measured value ofeff is 0.006 66, with 7.88% uncertainty, of which the deviation is 2.15% compared with the theoretical value. Because of the good matching between the measured value and the theoretical value, the experiment method is demonstrated to be effective.