I have tried in this paper to make it clear, in the first place, that the principle of universalizability plays the role of a criterion which demarcates between moral judgements and non- moral judgements in R. M. Hare's moral theory; and to clarify, in the second place, that his principle of universalizability has three peculiar features such as: (1) whether or not an action is universalizable depends on a person's desire for the action; (2) Hare's principle of universalizability implies reversibility; (3) when we universalize the action, we must give weight to other's desires as if they were our own. It seems to me to follow from the above mentioned three features that Hare's principle of universalizability leads to a sort of utilitarianism which aims at satisfying desires of all the parties concerned. Now if his principle leads to a sort of utilitarianism, then we might be able to think that his moral theory is naturalistic. However, I argue that his theory is not naturalistic, for Hare insists that an action cannot be moral unless a person judging the action prescribes the action.