A phase detector, arranged for comparing a phase of a first clock with a phase of a second clock. The phase detector includes a phase detection stage and a metastable prevention stage. The phase detection stage is arranged to receive the first clock and the second clock, and to output a phase comparison result in accordance with the phase of the first clock and the phase of the second clock. The metastable prevention stage is arranged to receive the phase comparison result, and to output a stable phase comparison result in accordance with the phase comparison result.