An aluminum nitride powder having the surface treated with an organosilane represented by formula, R.sup.I.sub.a R.sup.II.sub.b SiY.sub.4-1-b, and/or a partial hydrolysis condensate thereof to acquire excellent moisture-proof, wherein R.sup.I represents a 6-20C alkyl group or a group formed by substituting halogen atom or atoms for part or all of the hydrogen atoms attached to carbon atoms of the 6-20C alkyl group, R.sup.II represents a 1-20C hydrocarbon group or a group formed by substituting halogen atom or atoms for part or all of the hydrogen atoms attached to carbon atoms of the 1-20C hydrocarbon group, Y represents a hydrolyzable group, a is an integer of 1 to 3 and b is an integer of 0 to 2, provided that a+b is an integer of 1 to 3; and a thermally conductive grease composition comprising (A) 50-95 weight % of the aforementioned surface-treated aluminum nitride powder and (B) 5 to 50 weight % of at least one base oil selected from the group consisting of liquid silicones, liquid hydrocarbons and fluorohydrocarbon oils, and further, if desired, (C) 0-30 weight % of a thicknener selected from the group consisting of zinc oxide, alumina, boron nitride and silicon carbide powders.