A surface modification method comprising bringing, into a high-temperature flame formed by use of a combustible gas and a combustion-supporting gas, inorganic oxide powder A having an average particle size falling within a range of 0.5 to 200 .mu.m as measured by means of laser diffraction/scattering particle size analysis and inorganic oxide powder B having a particle size calculated on the basis of its BET specific surface area of 100 nm or less, to thereby modify the surfaces of particles of the powder A by means of the particles of the powder B. A surface modification method comprising bringing, into a high-temperature flame formed by use of a combustible gas and a combustion-supporting gas, the inorganic oxide powder A; and bringing again the resultant powder into the high-temperature flame, to thereby modify the surfaces of particles of powder A.