A process for producing a hydrophobic silica powder, comprises the steps for hydrophobic treatment of: adding to a silica sol in mixed solvent having a silica concentration of 5 to 50 mass % obtained by mixing an aqueous silica sol containing hydrophilic colloidal silica having a specific surface area of 5.5 to 550 m.sup.2/g with a hydrophilic organic solvent in a mass ratio of 0.12 to 2.5 based on water in the aqueous silica sol, a disilazane compound of formula (1) (R.sup.1.sub.3Si).sub.2NH (1) wherein each R.sup.1 is C.sub.1-6alkyl group or phenyl group that is selected independently of one another, in an amount of 0.1 to 20 mmol per surface area 100 m.sup.2 of the hydrophilic colloidal silica, to obtain a mixture of the silica sol in mixed solvent and the disilazane compound; and heating the mixture at a temperature of 50 to 100.degree. C. for aging it to obtain a slurry dispersion of hydrophobic treated colloidal silica. The process provides a hydrophobic silica powder through a simple hydrophobic treatment step.