Abstract: In Being and Nothingness, Sartre returned to the problem of ontology by the way of phenomenology, which is about being. He considered that the problem of being did not mean ‘being is being’, which was the basical and esscencial being of all the beings ,but it was ‘ the being of the appearance’, which was the existent way of the phenomenon and how it appeared. That is his special phenomenology. Sartre’s phenomenological ontology is deferent from the traditional one, because it is both phenomenological and ontological, which is the ontology as phenomenology. Sartre difererated two types of being in the phenomenological transform of traditional ontology, namely, the in-itself and for-itself. The in-itself is ‘being what it is’, while the for-itself is ‘being what it is not and not being what it is’. There is the significance or ability of nihilation in the ontological structure of the for-itself , which can make the being what it is to be the being what it is not, or the being what it is not to be the being what it is, and it comes from the original conscious as ‘nothingness’. The original conscious as ‘nothingness’ is the foundation that makes the for-itself pretence to self and the internal negation possible, and it can make the in-itself revealed and make two independent regions between the in-itself and for-itself can communicate. Above all ,it is the ontological source of the absolute freedom of the for-itself.