A transmitter device with I/Q mismatch compensation and a method thereof are provided. The transmitter device includes a transmitter circuit, a loop-back circuit and a baseband circuit. The transmitter circuit is configured to convert a baseband signal into an RF signal based on a specific gain configuration of a plurality of gain configurations. The loop-back circuit is configured to retrieve the RF signal from the transmitter circuit. The baseband circuit is configured to compensate the baseband signal by specific transmitter I/Q mismatch corresponding to the specific gain configuration, wherein a plurality of transmitter IQ mismatches are determined for the plurality of gain configurations by the retrieved RF signal, and the specific transmitter I/Q mismatch among the plurality of transmitter IQ mismatches is determined by the retrieved RF signal, which is converted in response to the specific gain configuration, from the loop-back circuit.