Precipitate formation behavior in high-level liquid waste (HLLW) and its filtration characteristics were examined experimentally, using a simulated HLLW. The amount of precipitate formed by denitration became minimum, only at about 5% of Mo, Zr, Te and Ru, if the simulated HLLW was pre-heated until the total heat input exceeded 7.9·106 J/I HLLW before denitration or denitrated with the total heat input of more than 1.1·107 J/I HLLW. Under these conditions, a needle-shaped precipitate with 0.51.0 m diameter and 35 m length was formed. This precipitate can be separated easily by vacuum filtration. While, fine particles of about 0.1 m diameter were precipitated during denitration, if the simulated HLLW was denitrated under the conditions the amount of newly formed precipitate was not minimum. It was difficult to separate the fine particles by vacuum filtration.