The thermal effect of a loss of flow accident and afterheat to the MARS blanket are investigated. The temperature response of the first wall, as well as the whole blanket, is calculated with a finite difference method. For a loss of flow accident, the plasma has to be quenched within 10 to 35 seconds, beyond which loss of the first wall may occur. Active cooling will be required for the blanket for afterheat within the first day after the plasma is shut off. After the first day, the reflector will provide a heat sink for passive cooling of the blanket.