PURPOSE: To obtain a polyolefin composite powder excellent in adhesiveness and powder property, by partially or totally curing a liquid diolefin rubber applied on the surface of a polyolefin powder or infiltrated into its interior.

CONSTITUTION: A composite powder is formed by partially or totally curing a liquid diolefin rubber applied on the surface of a polyolefin powder or infiltrated into its interior. Or, a composite powder is formed by partially or totally curing by the copolymerization of a liquid diolefin rubber and a vinyl group-containing functional monomer which are applied on the surface of a polyolefin powder or infiltrated into its interior. The method of their formation consists of mixing a polyolefin powder with a liquid diolefin rubber and, if necessary, a vinyl group-containing functional monomer and a radical generating agent at a temperature below the softening point of the polyolefin powder under a condition in which the powder does not form agglomerates until the diolefin rubber is partially or totally cured.