A ring-type voltage-controlled oscillator with a good duty cycle for use in a PLL frequency synthesizer. The delay cell circuit used in the ring-type VCO comprises two first inverters, two resistance units, and a differential delay circuit. The inverters receive respective differential input signals and generate respective differential signals to resistance units. The differential delay circuit is coupled to the resistance units, generating differential output signals which are a delayed version of the differential input signals. The resistance units have a resistance value adjusted according to a resistance control voltage for controlling the strength of inverters so as to alter the time delay of the first and second differential output signals.