Abstract: In this paper, a highly sensitive waveguide-type high power microwave (HPM) detector using a vacuum diode is designed, manufactured and experimented, aiming at directly measuring the HPM pulses at X-band. It consists of a WR-90 waveguide, a 6D16D vacuum diode, a low-pass filter andthree tuning screws. Our experiments at 9.3 GHz show that the detector can detect about 40 dB greater pulsed microwave power up to 10 kW compared with crystal diodes, its response time is less than 2 ns, the large output signal from the detector is available (up to 7 V/kW), and it can reliably operate free from electromagnetic interference at free space power density more than 2 kW/cm2.