A bias circuit and a voltage-controlled oscillator (VCO) thereof suitable for improving the stability of the bias circuit are provided. The bias circuit includes: an error amplifier circuit, having an inverting input terminal connected to a reference voltage; a voltage-controlled current source, having a voltage control terminal connected to a voltage output terminal of the error amplifier circuit, in which a current generated by the current source is controlled by a voltage at the voltage output terminal of the error amplifier circuit; a delay control circuit, having a current input terminal connected to the voltage-controlled current source, an output terminal connected to a non-inverting input terminal of the error amplifier circuit, and a voltage input terminal connected to a supply terminal of the control voltage, and the delay control circuit is adapted to adjust an output voltage of the delay control circuit according to a control voltage.