The indentation toughness equation proposed by Anstis et al. was re-examined with a particular emphasis on the definition of the hardness parameter used. It was shown that, because of the existence of the well-known indentation size effect (ISE), the apparent hardness defined as the ratio of the applied load to the projected area of the resultant indentation usually varies with the applied load and seems not to be suitable for use in indentation toughness determination. A new equation for determining indentation toughness, in which a load-independent hardness number was incorporated, was proposed.