A phase locked loop (PLL) and a voltage controlled oscillator (VCO) thereof are provided. The VCO includes a ring oscillator circuit and a control circuit. The ring oscillator circuit is used for providing an output clock signal; and the control circuit is coupled to the ring oscillator circuit, and used for receiving an output voltage to respectively provide a first voltage-frequency gain and a second voltage-frequency gain so as to control a frequency of the output clock signal provided by the ring oscillator circuit, wherein the first voltage-frequency gain is larger than the second voltage-frequency gain.