A quadrupole mass spectrometer (QMS) with an energy analyzed electron source has been available for measurement of ionization efficiency curves (IE curves) by electron impact. Whole of the measurements were controlled by a computer system. The energy spread of an electron beam used was less than 60 meV full width at half maximum (FWHM). The ionization energies of furan, benzene, pyridine, and cyclohexane were determined by means of the square root fitting method of the ionization efficiency curves. The energy scale was calibrated by measuring the threshold value for argone ion which is known as 15.759 eV.