A method for calibrating a transmitter-to-receiver (T2R) relative phase in millimeter wave (mmWave) beamforming system includes: transmitting a first calibrated signal to a second antenna of the mmWave beamforming system through a first antenna of the mmWave beamforming system according to a first transmitter (TX) input signal; receiving the first calibrated signal through the second antenna, and obtaining a first loopback receiver (RX) signal according to the first calibrated signal received by the second antenna; transmitting a second calibrated signal to the first antenna through the second antenna according to a second TX input signal; receiving the second calibrated signal through the first antenna, and obtaining a second loopback RX signal according to the second calibrated signal received by the first antenna; and calibrating the T2R relative phase according to a phase difference between the first and second loopback RX signals.