A hydrophobic metal oxide powder obtained by simultaneously surface-treating a powder of a metal oxide (for example, silica, titania, alumina, or the like) with silicone oil having epoxy groups and a non-polymeric amine compound having a lower number of primary and/or secondary amino groups than the number required for reaction with all epoxy groups, or surface-treating the powder with silicon oil having epoxy groups and amino groups and previously obtained by reaction. Straight silicone oil may be further used for surface treatment. Frictional charge which is controlled within a wide range of from negative charge to positive charge, high hydrophobicity and fluidity can be imparted to the powder by using a relatively small amount of coating, and a hydrophobic metal oxide powder useful as an additive for an electrophotographic developer can be obtained.