A phase calibrating apparatus applied in a mixer includes a signal generator outputting a first signal and a second signal. The first signal has a phase differing by a phase value from the second signal. The phase calibrating apparatus is used to adjust the phase value to a predetermined value. The phase calibrating apparatus includes a multiplication unit and a comparator/controller unit. The multiplication unit has a multiplication of the first signal and the second signal to output a DC value corresponding to the phase value. The comparator/controller unit compares the DC value with a reference value and outputs a control signal. The signal generator adjusts the phase value according to the control signal until the phase value is substantially equal to the predetermined value.