A quadrature VCO includes a first oscillator unit and a second oscillator unit. Each of the first and second oscillator unit is composed of a DC bias source, a complementary cross-coupled pair, an LC resonator unit, a frequency-doubling sub-harmonic coupler unit, and a ground terminal. When the LC resonator units of the first and second oscillator units are operated, four signals of different phases can be outputted via the output terminals. In this way, the output phase difference of the two oscillator units can keep 180 degrees and allow the two oscillator units to mutually inject signals to generate quadrature output signals.