A pulsed ESR spectroscopy system was developed using the high-power millimeter wave of gyrotron. The FID signal of stable radicals of BDPA diluted with polystyrene was successfully observed by the quadrature detection method. By Fourier transforming of these FID signals, high resolution FT-ESR spectrum of BDPA was successfully obtained. An attempt was made to improve the sensitivity by averaging the obtained spectra. It was found that there was an n-fold relationship between the number of averagings n and the S/N ratio.