Abstract: The prompt neutron decay constant was measured by using reactor noise technology at delayed criticality for 300^# pool research reactor. The reactor core uses irradiated low-enrichment fuel loading. The prompt neutron decay constant is obtained from nonlinear least-squares fitting of the cross-power spectral densities measured between two detectors placed adjacent to the core. The measured prompt neutron decay constant α_c at delayed criticality with a power of 4 kW is (83.4±0.7) s^-1.