The present invention preferably provides a polishing composition for a hard disk substrate, containing aluminum oxide particles and water, wherein secondary particles of the aluminum oxide particles have a volume-median particle size of from 0.1 to 0.7 .mu.m, and particles having particle sizes of 1 .mu.m or more of the aluminum oxide particles are contained in an amount of 0.2% by weight or less of the polishing composition; and a method for manufacturing a hard disk substrate using the polishing composition. By using the polishing composition and the method for manufacturing a substrate of the present invention, for example, a hard disk substrate suitable for high recording density can be provided in high productivity.