The main aim of this paper is to show that in Leśniewski's Ontology, the law of Leibniz and a special case of Barbara which we refer to as the singular Barbara are equivalent to each other. To show this, we choose a group of theses of Ontology which are sufficient to establish the intended equivalence. From the result it follows that the problematic characters concerning the law of Leibniz reduce either to the validity of the singular Barbara or that of the theses used in establishing the intended equivalence.