This paper presents a detailed analysis on the relationship between quality factor (Q) and the amplitude of CMOS LC VCO. It is shown that controlled voltage amplitude is benefit for acquiring a large Q of the oscillator. Low power and phase noise of LC VCO can be optimized by tuning Q of tank. The analysis has been carried out both theoretically and by simulation for the design of a low power and low phase noise LC VCO in a standard 0.18μm CMOS process. The VCO with on-chip spiral inductor is operating from 2.9 to 3.9 GHz. The simulation phase noise level is -113 dBc/Hz at 1MHz offset frequency while dissipating only 2.4mWat a 1.8V power supply.