A clock generation circuit that is suitable for passive Ultra High Frequency Radio Frequency Identification(UHF RFID) tag chips is proposed. Different from the traditional relaxation oscillator structure，the N-Metal Oxide Semiconductor(NMOS) gate voltage is utilized as a comparison reference level，without the need of complex comparator circuit. The circuit structure is simplified，which greatly reduces the power consumption and reduces the layout area. At the same time，the NMOS transistor is adopted for temperature and process compensation，the gate voltage of which is utilized to control the charge and discharge current. The output frequency is stable without traditional clock calibration. The circuit is designed in a standard Semiconductor Manufacturing International Corporation(SMIC) 0.18 μm Complementary Metal Oxide Semiconductor(CMOS) technology，with the condition of 1 V power supply voltage and 130 nA reference current，and the power consumption of this circuit is only 447 nW. When the process corners are changed from ss to ff，the output frequency deviation is no more than 2.43%，and the temperature changed from -40 ℃to 90 ℃, the output frequency deviation is less than 2.62%，so the circuit is suitable for passive RFID tag chips usage.