Abstract: With the development of miniaturization and integration of radar system, the signal leakage from the transmitter to the receiver is more serious in FMCW radar. Signal leakage has become the key factor restricting the performance of FMCW radar system. The serious signal leakage will restrict the performance of FMCW radar system, and improving the isolation between transmitter and receiver has become an important part of system research. To solve this problem, a novel RF cancellation method to improve system isolation is proposed and experimentally demonstrated basing on the separable feature of FMCW radar in frequency domain between the leakage signal and the echo signal. The digital phase locking loop (PLL) is used in this method to produce the cancellation vector signal. By studying the relationship between the cancellation ratio and the phase error, amplitude error, a radio frequency cancellation scheme with adaptive function under the given initial value of the leakage signal is established. The proposed RF cancellation method achieves cancellation ratio better than 25 dB in the bandwidth of 1 GHz, and the RF cancellation circuit has the advantages of low power consumption and simple structure.