PURPOSE: To improve hydrophobic property by treating Al2O3, powder with a fluorine substd. silane compd. and organopolysiloxane each represented by a specified formula.

CONSTITUTION: A fluorine substd. silane compd. represented by formula I, organopolysiloxane represented by formula II and an amine catalyst are diluted with a solvent in a prescribed weight ratio to prepare a treating agent. In the formula I, R1 is 1-10C fluorine substd. alkyl, X is SO2NR4 (R4 is H or 1-5C alkyl), CONR4, CO2, or CH2O, R2 is 1-5C alkyl, R3 is Cl, Br or 1-5C alkoxy, (p) is 0 or 1, (q) is 1-10, (m) is 2-5 and (n) is 1-3. In the formula II, each of R and R6 is an org. substituent or hydrodiene, R5 and R6 are not simultaneously methyl, (a) is 0 or a positive integer, (b) is a positive integer, and in the case of (a)=0, R5 and R6 are not simultaneously hydrodiene. The treating agent is sprayed on Al2O3, powder having 50-200m2/g specific surface area and the powder is heated to 100-300°C to obtain Al2O3 powder having ≥50 hydrophobic degree and 0-40μm/g charges by triboelectric charge to iron.