In fundamental ontology, Heidegger argues that Dasein (man) whose mode of Being is defined as existence is essentially distinguished from things that exist in the mode of present-at-hand. But besides this distinction, there is another way to illustrate the difference between man and things. According to Heidegger, the self-identity of man is not equated with the self-identity of things. He argues that while the former is defined as the verbal self-identity, which he calls self-subsistence, the latter is characterized as the fixed and immutable self-identity, i.e., self-steadiness. Leibniz, however, also insists that the self-identity of monad is defined as an acting identity, insofar as it is grounded in the active force(vis). Hence the objective of this paper is to clarify a distinctive meaning of an acting self-identity of Dasein by comparing the acting self-identity of Dasein and the active force of monad in the philosophy of Leibniz. The self-identity (essence) of Dasein is understood in a verbal sense, for Dasein is determined in its potentiality-for-Being. So Heidegger seeks to elucidate the self-identity of Dasein in respect to potentiality-for-Being. However, when Heidegger speaks of a verbal self-identity based on potentiality-for-Being, he does not mean the potential (dynamis) self that strives for the full-fledged actuality. The potential self loses its potentialities when it reaches the actuality. But on contrary to this, the potentiality of Dasein that is not constricted by the actuality never gets diminished even when it is fulfilled in the state of actuality. In striving for the actuality, it becomes greater and greater. And this dynamic possibility of Dasein can be clearly understood when it is compared with the self-propulsive force (vis) in the philosophy of Leibniz. Although the dynamic possibility of Dasein and the concept of vis both presuppose the self-propulsive force that is not dependent on the external stimulus from other things, from the ontological perspective they are still different. For Heidegger, Leibnizian vis is based on the simple and unitary substance, which is closely bound up with the actuality. Hence, the active vis is unable to comport itself toward the domain of possibility that is absolutely beyond the realm of actuality. In contrast to the simple foundation of vis, the potentiality of Dasein is grounded in the multiple moments that are constituted in having-been, present, and future. And these multiple moments are never reduced to a unity but remain split. Since the potentiality of Dasein lies in the multiple moments, it is able to project itself toward the domain of possibility that always persists a measureless possibility. Existing in this measureless possibility, the self-identity of Dasein is in each case mine (je meines) rather than a universal self-identity.