The two power peaks during the Chernobyl accident were analyzed using the system thermal-hydraulic code RETRAN-02. The time and magnitude of the first power peak predicted by the RETRAN model compared well with the data presented by the Soviets. The analysis also revealed that one of the contributing factors to the second power peak was the depressurization of the system. Depressurization occurred upon rupture of the pressure boundary, which was caused by the first power peak. The depressurization of the system generated more voids, resulting in additional reactivity insertion, which produced a second peak. A parametric study showed that the positive reactivity introduced by the scram rods and the reactivity caused by the positive void coefficient were both important in contributing to the accident.