To obtain a lubricating oil which is free from sediment or layer separation and is excellent in lubricity in a wide temp. range by dispersing a fine hexagonal boron nitride powder having a specified average particle size in a lubricating oil.

A hexagonal born nitride powder which has an average particle size of 20-200nm and the content of which particles having particle sizes in the range of the average particle size ±30% is 80wt.% or higher is dispersed in a lubricating oil. The powder is obtd. e.g. by mixing a boron compd., a nitrogen compd., and a carbonate, sulfate, or nitrate of an alkali or alkaline earth metal and heating the resultant mixture at 1,000-1,500°C in a nonoxidative gas atmosphere or by thermally reacting a melamine compd. with boric acid or boron oxide in a solvent, cooling the reaction mixture to precipitate a fibrous compd., treating the fibrous compd. at 400-800°C in a nonoxidative atmosphere, pulverizing the treated compd., and treating the resultant powder at 1,000-1,800°C. A suitable amt. of the powder dispersed in a lubricating oil is about 0.01-30wt.%.