-This paper describes a new prediction method of tuning curves of a LC-tank voltage-controlled oscillator (VCO) with period calculation technique. With this period calculation technique, the prediction of oscillator tuning curves is more accurate compared with the traditional harmonic approxi -mation. The theoretical analyses are experimentally validated with a CMOS complementary LC-tank VCO implemented in 0.35µm 1P4M pure logic CMOS process.