This paper presents a systemic analysis for phase noise performances of differential cross-coupled LC oscillators by using Hajimiri and Lee's model. The effective impulse sensitivity functions (ISF) for each noise source in the oscillator is mathematically derived. According to these effective ISFs, the phase noise contribution from each device is figured out, and phase noise contributions from the device noise in the vicinity of the integer multiples of the resonant frequency, weighted by the Fourier coefficients of the effective ISF, are also calculated. The explicit closed-form expression for phase noise of the oscillator is definitely determined. The validity of the phase noise analysis is verified by good simulation agreement.