A highly efficient and bias-free inverse kinetics technique has been developed to determine the subcriticality of a nuclear reactor. The new algorithm has been tested through simulation studies against two other algorithms currently being used. It was also extensively tested by analyzing actual rod-drop experimental data taken at the Argonne National Laboratory. Both simulation studies and the real data analyses showed that the new algorithm proposed here is a superior method in estimating the subcriticality of a system from the time-dependent neutron flux transient data.