A hydrophobic fumed silica treated with a cyclic dimethylsiloxane, the hydrophobic fumed silica having an M-value representing an oleophilic degree in a range of 48 to 65, a tapping bulk density of not smaller than 80 g/L but not larger than 130 g/L, and an n-value representing the dispersion of 3.0 to 3.5 as measured in toluene. The hydrophobic fumed silica features a high bulk density while exhibiting a high hydrophobic property, the powder thereof capable of being favorably handled, being easily mixed in a matrix such as of a resin within short periods of time, and, further, being dispersed in the matrix to a high degree.