Power line (PL) interference is one significant artifact in elec?trocardiography (ECG) that needs to be reduced to ensure ac?curate recording of cardiac signals. Because PL interference is non?stationary and has varying frequency, phase, and ampli?tude in ECG measurement, adaptive techniques are often nec?essary to track and cancel the interference. In this paper we present a phase ? locked loop (PLL) ? based adaptive filter to cancel PL interference. The PLL obtains the reference signal that is fed into the adaptive filter to remove the PL interfer?ence at the central frequency of 50 Hz. It is found that the technique can effectively cancel PL interference in real ECG signals and, when compared with some existing techniques such as least mean squares (LMS) adaptive filter, the new technique produces better results in terms of signal?to?interfer?ence ratio (SIR).