Abstract: The leaked neutron multiplication constant of a fast critical assembly was measured by a long BF₃ counter to analysis the neutron density increase rate with time when reactivity was inserted. The neutron density increased rapidly, and became stable after 180 and 450 s respectively for the reactivity of ρ=-2.27×10^-3 and -4.79×10^-4 when reactivity was inserted. Compared with the calculated neutron density increase rate on the assumption of single group of delayed neutron, the measured rate is a little faster. Even so, the delay of neutron density increase must be considered when startup a reactor.