An I/Q imbalance calibration apparatus for a transmitter system operated in an I/Q imbalance calibration mode, the I/Q imbalance calibration apparatus comprises: a calibration signal generator, used to selectively generate a first in-phase calibration signal, a first quadrature calibration signal, or both of the first in-phase and quadrature calibration signals; an I/Q imbalance calibrator, electrically connected to the calibration signal generator, used to perform a I/Q gain imbalance compensation on the first in-phase and quadrature calibration signals to generate a second in-phase and quadrature calibration signals after receiving an I/Q gain imbalance, and selectively output the first in-phase calibration signal, the first quadrature calibration signal, the second in-phase calibration signal or both of the second in-phase and quadrature calibration signals to a front-end circuit of the transmitter system; a signal strength acquiring circuit, electrically connected to the front-end circuit of the transmitter system, used to selectively acquire and output one of a first through fourth calibration signal strengths, wherein the first through fourth calibration signal strengths are corresponding to a first through fourth calibration signals respectively, and the first through fourth calibration signals are processed signals which the front-end circuit processes the first in-phase calibration signal, the first quadrature calibration signal, the second in-phase calibration signal, and both of the second in-phase and quadrature calibration signals respectively; and an I/Q imbalance estimator, electrically connected to the signal strength acquiring circuit, used to selectively estimate one of the first through fourth calibration signal strengths by a delta estimation, calculate the I/Q gain imbalance according to an estimated first and second calibration signal strengths, calculate an I/Q phase imbalance according to an estimated third and fourth calibration signal strengths. a calibration signal generator, used to selectively generate a first in-phase calibration signal, a first quadrature calibration signal, or both of the first in-phase and quadrature calibration signals; an I/Q imbalance calibrator, electrically connected to the calibration signal generator, used to perform a I/Q gain imbalance compensation on the first in-phase and quadrature calibration signals to generate a second in-phase and quadrature calibration signals after receiving an I/Q gain imbalance, and selectively output the first in-phase calibration signal, the first quadrature calibration signal, the second in-phase calibration signal or both of the second in-phase and quadrature calibration signals to a front-end circuit of the transmitter system; a signal strength acquiring circuit, electrically connected to the front-end circuit of the transmitter system, used to selectively acquire and output one of a first through fourth calibration signal strengths, wherein the first through fourth calibration signal strengths are corresponding to a first through fourth calibration signals respectively, and the first through fourth calibration signals are processed signals which the front-end circuit processes the first in-phase calibration signal, the first quadrature calibration signal, the second in-phase calibration signal, and both of the second in-phase and quadrature calibration signals respectively; and an I/Q imbalance estimator, electrically connected to the signal strength acquiring circuit, used to selectively estimate one of the first through fourth calibration signal strengths by a delta estimation, calculate the I/Q gain imbalance according to an estimated first and second calibration signal strengths, calculate an I/Q phase imbalance according to an estimated third and fourth calibration signal strengths.