A polishing composition for a hard disk substrate includes alumina particles, silica particles, and water. The volume median diameter of secondary particles of the alumina particles measured by a laser beam diffraction method is 0.1 to 0.8 .mu.m. The volume median diameter of primary particles of the silica particles measured by transmission electron microscope observation is 40 to 150 nm. The standard deviation in number-basis particle size of the primary particles of the silica particles measured by the transmission electron microscope observation is 11 to 35 nm. The polishing composition for a hard disk substrate can preferably reduce the embedding of alumina abrasive grains into the substrate without impairing the productivity.