In this study, comprehension of being in fundamental ontology which is the essence of Heidegger metaphysics and the connection of the thinking structure behind this comprehension with the principle of identity shall be analyzed. The basic purpose of this study is to assess the problem of ousia based comprehension of being of Western metaphysics which Heidegger tried to go beyond on the basis of comprehension of identity of being and Dasein in Heidegger s philosophy. Through fundamental ontology Heidegger aims at subjecting comprehension of being to deconstruction which comprehension is left by Western metaphysics tradition to be forgotten. In this context Heidegger expresses that there are two different comprehensions of being in Western metaphysics tradition: the first one is the comprehension of being created by Ancient Greek thinking structure which comprehends being as the one to exist there with phusis; the second one is the comprehension of being which breaks off the comprehension of being of Ancient Greek metaphysics which is the one existing there and which transforms being into existing one through ousia. While the first tradition is particularly represented by Heraclitus among Ancient Greek philosophers, the second tradition is represented by Aristotle. Heidegger bases the problem of forgottenness of being in the history of Western metaphysics on the categorical thinking structure created by Aristotle s comprehension of being. This thinking structure breaks off being from its identity with thinking and transforms being into the existing one through ousia and transforms thinking to representation thereof. According to Heidegger, breaking off being which is phusis from thinking through ousia is at the same time the basis of forgottenness of being. For this reason the historical traces of the thinking structure which is responsible for forgottenness of being in fundamental ontology are subjected to deconstruction and the being is revealed once more with its foundations in Ancient Greek thinking. According to Heidegger, the being was forgotten by being transformed into the existing one through the principles of categorical thinking in the history of Western metaphysics, particularly in port- Aristotle philosophy tradition. And forgetting the being means leaving the connection between being and thinking because when the being is comprehended as the existing one , being gains existence independently from thinking and thinking transforms into representation of being. Nevertheless for Heidegger, the being is not the existing one but it is rather the being of Dasein in orientation to the one which was there all the time. Dasein is the thing that will give the meaning of the being since it is the only existing one which poses its own existence among all existing things as a problem because Dasein is the being which is in an orientation all the time with existentiality of the non- existing one . Dasein s orientation toward its being that exists there is its temporality belonging to its existentiality. Dasein is the existing one which will reveal the being from its forgottenness caused by categorical thinking with its temporal existence. The meaning of the being broken off from thinking with categorical principles is reconstructed in Heidegger s philosophy by making Dasein and property of the being belong to one another with concepts including nothing , temporality ,worldliness , spatialness , concern , other , being within the world etc. The being and Dasein in the comprehension of being constructed with those concepts is made identical with thinking and language in the property of each other. In this context while the manner of constructing identity of Dasein and the being is analyzed using conceptual connections in this study, at the same time the dependency of the thinking structure that forms those conceptual connections on the principle of identity shall be analyzed as well. Thus it shall be demonstrated that in Heidegger s fundamental ontology identity of Dasein and the being reconstructs the being or more fundamentally thinking but imposed a temporalessence to the existentiality of the being or thinking and sustained dependency on the principles of identity of the same tradition.