Oscillation amplitude is a key factor for NMOS/PMOS complementary cross-coupled LC-tank oscillator, and its proper estimation is beneficial to design oscillator and select process. An ordinary expression for oscillation amplitude of NMOS/PMOS complementary crosscoupled LC tank oscillator is derived by solving ShichmanHodge equation using Fourier Transform. It mainly depends on power supply voltage and threshold voltage. The effects of power supply voltages, transistor's sizes and inductor values are discussed. The calculation and Cadence simulation of oscillation amplitude show excellent agreement with error less than 5%.