A new type of interference selective amplitude modulation spectrometer is discribed in this paper. The diffraction grating illuminated by a collimated incident beam is placed in front of a Twyman interferometer. The beam is splitted into two parts and reflected by two mirrors acting as two diffraction gratings. Then the interference fringe is modulated. The theoretical analysis of flux and resolution is made. Using rectangle diaphragm the resolution of 1. 35 times of the grating is obtained. The apodization technique with elliptical diaphragm is discussed.