Abstract: A Variable high power directional coupler for S band is described in this paper.The coupler consists of two 3 dB couplers mounted in cascade and a phase shifter.inserted between the connecting legs of the 3 dB couplers. The basic principles,theoretical calculations and test results are presented. When the coupling factor isadjustable within 25 dB, the input voltage standing wave ratio of the system isnot greater than 1.20 for the frequencies of 2856±10 MHz. The tests have beenconducted successfully with a peak power of 15 MW and an average power of10 kW. The experimental results show that the system is possessed of widervariable range of the power, higher power-handling capacity and better vacuumperformance. The variable high-power directional coupler could be used in the high power RF systems for linacs and the other devices.