Over the last ten years, the progress of Nanoelectromechanical systems (NEMS) fabrication has opened new possibilities for the use of NEMS as high-performance, low-cost, compact sensors [1]. In NEMS resonators-based sensors, the resonant frequency depends upon the physical parameter to be measured [2]. To track dynamically the time-varying resonant frequency of the NEMS, one approach consists in embedding the NEMS in a self-oscillating loop so that it oscillates at its resonant frequency. The resonator chosen in this study uses electrostatic actuation and capacitive detection. This has the advantage of having a relatively simple implementation and good transduction efficiency. Furthermore, this actuation/detection scheme makes possible the monolithical integration of the NEMS resonator with a dedicated CMOS circuit. Co-integrated sensors have higher signalto-noise ratios since parasitic capacitances are drastically reduced. Examples of cointegrated NEMS resonators are presented in [3-5]. The main contribution of this work is the design of a NEMS/CMOS oscillator (NEMS as resonant element, CMOS circuit as sustaining amplifier) with a very simple amplifying circuit based on a single active transistor. The design of the electronics and the oscillator steady-state response are theoretically studied. The micro/nanomechanical resonator is described in section 2, the oscillation conditions and the CMOS sustaining amplifier are respectively analyzed in sections 3 and 4. Finally, the steady-state regime of the oscillator is determined by using a describing function method in section 5.