A power mixing circuit capable of maintaining a stable output voltage in a deep-power-down mode is provided. The power mixing circuit includes an input buffer, a power mixing control circuit, a power mixing driver and an output buffer. The input buffer is configured to operate using a first supply voltage, and to generate a first voltage signal in response to an input signal. The power mixing control circuit is configured to generate a power mixing control signal based on a power-up signal and a deep-power-down mode signal. The power mixing driver is configured to operate using an external supply voltage and a second supply voltage, to perform power mixing on the external supply voltage and the second supply voltage, and to generate a second voltage signal. The output buffer is configured to operate using the second supply voltage, and to generate an output signal.