[en] In a method of estimating a performance of a reactor core, an approximate value of an effective multiplication factor determined by a nuclear thermohydrodynamic calculation is compared with an aimed value of previously provided effective multiplication factor. Insertion depth for control rods is corrected repeatingly so that the approximate value is converged to the aimed value. Further, it is judged whether the insertion depth for the control rods can be realized or not as an insertion depth for control rods of an actual reactor when the approximate value is converged to the aimed value. If it is impossible, a depth nearest to the insertion depth for the control rods and realizable by using an actual reactor is calculated. Nuclear thermohydrodynamic calculations are repeated to determine a power distribution in the reactor core and an effective multiplication factor of the reactor core by using the insertion depth for control rods, reactor core thermal power and reactor core flow rate as parameters. The approximate value obtained by the nuclear thermohydrodynamic calculation is compared with the aimed value, and the reactor core thermal power or the reactor core flow rate is corrected such that it is converged by repetition to the aimed value. With such procedures, the insertion depth for the control rods can be estimated easily in a short period of time. (N.H.)