A voltage controlled oscillator(VCO) is designed and implemented at X-band using Gunn diode and Varactor diode on a microstrip planar circuit board. A radial transmission line is used as a resonant part, which is coupled to a load and a varactor diode part through microstrip. The coupling is modelled with transformer and its coupling coefficient is calculated from experimental data. This VCO has a frequency response about 60MHz in 15V control voltage range and 11dBm output power.