There is provided an adaptive loop gain control circuit for a voltage-controlled oscillator (VCO). The adaptive loop gain control circuit for a voltage-controlled oscillator (VCO) includes a detected voltage generating unit which generates a detected voltage signal according to changes in an operating voltage and an operating temperature, and a control circuit unit which outputs an oscillation control current signal according to the detected voltage signal and an input control voltage signal. The adaptive loop gain control circuit for a voltage-controlled oscillator (VCO) compensates for an oscillation control current according to changes in operating voltage and temperature and compensates for the gain of a phase locked loop (PLL) system, thereby ensuring high operating stability in the PLL circuit.