Heidegger and Sartre developed the projects of their fundamental ontologies within the framework of the phenomenological approach. The traditional view of reality is based on dualistic oppositions of ideal and material, spirit and body, reality and possibility, and visibility and essence. According to both authors, phenomenology enables elimination of the above-mentioned dualisms and restoration of the world’s ontological unity on a reliable foundation. A special attention is paid to a problem of authenticity and transcendence from the point of view of above-mentioned ontological concepts. The problem of authenticity is one of the main issues of human life: the person can come up against a situation of a tough choice and get to a special paradigm of submission that it is practically similar to loss of own authenticity. Method of a research is the comparative analysis of approaches of Sartre and Heidegger to phenomena of freedom, authenticity, the truth and being. In addition, the analysis of a modern discussion on this subject is carried out. The main conclusion of this research is that though Sartre's existentialism was exposed to criticism both from right, and from the left intellectuals, and is not a "fashionable" current at present, and the fundamental ontology of Heidegger was estimated by Levinas as ontology of the power subject-centered line, nevertheless the author considers that attention which is paid in these concepts to such phenomena as the voice of conscience, care and freedom represents the most urgent philosophical value nowadays; the author in this regard points to insufficient validity of modern criticism of approaches of Heidegger and Sartre, in particular - reproaches in a subject-centered line from adherents of "ontology of the Other"