The laser diffractometer is an effective instrument for calibrating pitch standard of a grating structure. A conventional diffractometer based on the Littrow configuration cannot measure a grating whose pitch is less than half of the laser wavelength when the diffractometer is operated in the atmosphere. This study proposes an immersion diffractometer to raise the refractive index of the environment. Thus the new approach can overcome the limit of one-half wavelength. A 288 nm grating was measured using an immersion diffractometer with a 633 nm laser and using a conventional diffractometer with a 543 nm laser to demonstrate the feasibility and effectiveness of the proposed technology. The difference between the pitches obtained by these two methods is around 0.05 nm.