This paper presents an active capacitance circuit using a LC series-feedback scheme. The noise performance is excellent and the good stability is accomplished by adjusting the feedback elements. We analyze the circuit performances in detail and apply it to a voltage controlled oscillator. The active capacitance circuit provides negative resistance that can compensate not only the loss of varactor diode but also the parasitic resistance of inductor. A VCO employing the active capacitance circuit is designed at 2. 179-2.191 GHz and the phase noise is -104.67dBc/Hz at 10 kHz offset within the tuning range.