A ratio type of capacitance measurement circuit is proposed to measure an extremely small value of the fF capacitance on this paper. This measurement circuit is formed with a switched-capacitor integrator, a comparator, and logic circuit blocks to control the switches. It converts the measured ratio value between the known value of on-chip capacitor and the unknown value of capacitor to the digital signal. The fF capacitance with minimized error can be obtained by calculating this ratio. This proposed circuit is designed with standard CMOS $0.18{mu}m$ process, and various HSpice simulations prove that this capacitance measurement circuit is able to measure the capacitance under 5fF with less than ${pm}0.3%$ error rate.