This paper presents the radio frequency (RF) transmitter using pulsewidth modulation (PWM) method. The proposed transmitter can perform with lower power consumption compared to other RF transmitters using a conventional ring oscillator because the proposed voltage-controlled oscillator (VCO) is operated by ON-/OFF-time of the PWM signal. Moreover, the proposed VCO with a metal-oxide semiconductor switch can reduce the power consumption significantly due to the reduction of idle current loss. The power dissipation of the transmitter is measured as 0.75 mW at a carrier of 315 MHz. The transmission of the modulated signal is performed by the on-chip antenna with bond-wire inductor. The on-chip antenna has gain of -34.33 dBi. The radiated power is measured as -32.3 dBm at a distance of 10 cm between the receiver and the transmitter.