The present invention discloses method for manufacturing device isolation film wherein a high selectivity slurry containing M.sub.xP.sub.yO.sub.z is used for polishing nitride film to prevent the generation of moat. In accordance with the method, a pad oxide film and a pad nitride film formed on a semiconductor substrate and the semiconductor substrate are etched to form a trench. A liner nitride film and an oxide film for device isolation film filling the trench are formed on the entire surface. The oxide film for device isolation film is first etched using a low selectivity slurry, and further etched using a high selectivity slurry to expose the liner nitride film. The liner nitride film is polished using a high selectivity slurry containing M.sub.xP.sub.yO.sub.z and the pad nitride film is then removed.