A device and method for automatically controlling power of a radio mobile terminal connected to a hands-free kit and for preventing the radio mobile terminal from malfunctioning. In a power control method for automatically turning on a radio mobile terminal connected to a hands-free kit, the radio mobile terminal including a power control signal generator, a signal output terminal and a signal input terminal, the hands-free kit including a power-on signal generator and a signal detector, a power-on signal is supplied to the signal input terminal for a predetermined time period through the power-on signal generator when the radio mobile terminal is connected to the hands-free kit. After the predetermined time period elapses, generating the power-on signal is stopped. It is determined if the driving signal generated by the signal output terminal is detected through the signal detector. If the driving signal is detected, generating the power-on signal is stopped.