The line narrowing of excimer lasers is discussed. The theory for an optical two-effect intracavity line narrowing device, the multipass grating interferometer (MGI), is presented. An MGI contains a grating aligned in its second-order Littrow configuration and a mirror aligned parallel to the grating surface reflecting back the beam normal to the grating corresponding to the first-order diffraction. The Littrow grating is doing the coarse line narrowing, and the mirror aligned parallel to the grating has similar line narrowing properties as tilted intracavity Fabry-Perot etalons. An MGI is applied to a KrF laser cavity to achieve a linewidth of 0.03 cm−1.