In Letter on Humanism, Heidegger's critique to Human has a two-fold meaning. On one hand, his critique of Sartre's existentialism was his self-critique to the Metaphysics of Dasein in Being and Time. On the other hand, it was also his self-reflection to the affinity between the existentialist philosophy of history and Nazi's ideology of nationalism. After examining Heidegger's critique to Humanism, his implicit critique to Nazi's ideology can be discovered. Thus later Heidegger's philosophy should be conceived as an attempt to overcome his early philosophy and its affinity to Nazi's ideology.