In this paper, We analyze Colpitts type voltage contrqlled oscillator(VCO) used in personal handhel phone using a ninlinear analysis with third-order model. The result shows the non-exponentially decaying shifting bias superimposed on the oscillator output which is different with the exponentially decaying shifting bias from the linear analysis. The stable oscillation criterion during a frequency change in a design of VCO can be also deermined using proposed non-linear analysis. The theory is confirmed using PSPICE simulation and the experimental result of GaAs VCO mathed very well with the theory.