The absolute calibration factor of extreme ultraviolet spectroscopic instrument which has recently been determined from absolute radiation profile measurement of bremsstrahlung continuum has been investigated by comparing the calculated diffraction efficiency of grating. An overall tendency of the wavelength dependence of the calibration factor from 40 Å to 500 Å can be reproduced by that of the grating efficiency, especially the agreement between the measured calibration factor and the calculated grating efficiency has been found to be fairly good for the wavelength range 200 Å-500 Å.