PURPOSE: To improve quantity of electrostatic charge and to permit easy control of an electrostatic charging characteristic by using a specific silane coupling agent on the surface of magnetic powder to form a coating material thereon.

CONSTITUTION: A coatong material is formed on the surface of magnetic powder by using the silane coupling agent expressed by the formula. The formation of the coating material of this invention on the magnetic powder is accomplished by dissolving the silane coupling agent in a suitable solvent, mixing the magnetic powder therewith and bringing the mixture into reaction. A frictional electrostatic charging characteristic is then easily controlled, and an image having a good gradient characteristic is obtd. In the formula, R1 denotes a substd. or unsubstd. alkyl group, R2 denotes an unsubstd. lower alkyl group, R3 denotes an alkyl group having at least one univalent or bivalent group, unsubstd. alkyl group or substd. or unsubstd. vinyl group selected from the group consisting of an epoxy group, mercapto group, amino group, vinyl group which may have a side chain, carboxy group, formyl group, oxy group, imono group, carbonyl group, vinylene group which may have a side chain, and an aryl group. m is 2 or 3.