In this paper, RF models for an MOS varactor and an MIM capacitor fabricated in 0.18-mum CMOS technology were developed in the GHz frequency range. In particular, the MOS varactor and the MIM capacitor were modeled in order to predict the behaviors of those with various bias conditions and device dimensions at high frequency, respectively. As a result, the RF models produced results that were in very good agreement with measurements. These models will be very valuable to the RF circuit designers because they can accurately describe the characteristics of MOS varactors and MIM capacitors.