Theory and experiments are reported of the use of a waveguide with a special helical corrugation of its inner surface to provide pulse compression of frequency modulated pulses. Such waveguide has some important advantages over a conventional smooth waveguide as a dispersive delay line for compression of frequency modulated microwave pulses. A compressor using a helically corrugated waveguide can provide generation of nanosecond multi‐gigawatt microwave pulses. Results are presented of experiments at kilowatt power levels that are in good agreement with the theory. The principle exploited in this type of pulse compression is well known and used widely in radar applications. It is based on the propagation of a quasi‐monochromatic wave packet which has a slow (on a period scale) frequency modulation through a dispersive medium, for which the wave group velocity is a function of frequency. For the novel microwave pulse compressor the wave dispersion was synthesized by coupling a TE2,1mode and a counter‐rotating TE1,1mode as the near and far from cutoff modes, respectively, on a three fold helical corrugation in the wall of a cylindrical copper waveguide. A frequency swept pulse of 1kW peak power was used as the input source to the helical waveguide compressor. Experimental optimization of the start and the end frequencies as well as the input pulse duration resulted in measured peak output powers of over 10kW and a maximum compression ratio of 10.9 (10.4dB) with an efficiency of 44&percent;. These results including the output pulse shape were in very good agreement with the simulations. The simulations predict that with further optimization of the frequency modulation the power compression ratio and efficiency can be increased to 18.7 and 65&percent; respectively. A frequency modulation similar to that used here can be realized at the falling edge of a microwave pulse produced from a high‐power (∼1 GW) BWO, indicating the potential of this type of compressor for producing multi‐gigawatt output pulses. © 2003 American Institute of Physics