This thesis presents a 24GHz microwave down-converting mixer for K-band Frequency Modulated Continuous Wave (FMCW) Radar System which is realized in 0.18μm CMOS process. At circuit design level, due to the difficulty of acquiring power for high frequency local oscillating signal, reducing the optimal operating power level is a priority. In addition, high symmetrical layout technique, application of interference rejecting transmission lines, and the choice of the schematic in mixer all contribute to outstanding isolation performance. Complementary-conducting-strips (CCS) transmission line, Metal-Insulator-Metal (MIM) capacitor, and Metal-Finger Capacitor (MFC) are used as passive components.