The parallel velocity distribution function of the weakly relativistic electron beam of a quasi-optical gyrotron has been determined by measuring the Doppler-shifted Electron Cyclotron Emission (ECE) at an angle theta=15 degrees with respect to the external magnetic de-field. Due to the Doppler shift, the frequency of the spontaneous cyclotron emission at the fundamental (nu(0) = 100 GHz) is upshifted to 140 GHz. A broadening of the spectrum up to 10 GHz [full width at half maximium (FWHM)] was measured. The measured mean frequency agrees well with the theoretical predictions, but the observed line-width, and hence the parallel velocity distribution function, is 2-3 times larger than expected. Considerations on ECE-measurements of the electron beam energy spread, performed at larger angles theta, are also discussed. (C) 1996 American Institute of Physics.