A voltage mode controlled liner frequency modulation oscillator comprises a voltage modulation circuit, a reference current generating circuit, and an oscillating circuit. The voltage modulation circuit is configured to generate a modulation voltage according to a feedback voltage and a first reference voltage. The reference current generating circuit, coupled to the voltage modulation circuit, is configured to generate a first reference current according to the modulation voltage and a second reference voltage. The oscillating circuit, coupled to the reference current generating circuit, is configured to generate an oscillating signal with an oscillating frequency according to the first reference current, wherein the oscillating frequency varies with the modulation voltage.