This article considers the relation of reason and faith in Kierkegaard's existentialist Thought. Kierkegaard describes the faith as a kind of "passion", "leap" or "relation to God" and believes that it is over reason and sometimes has position that is completely against reason. According to Kierkegaard, the faith can save us but we cannot know the faith by reason. He defend faith against reason, hence he is called a "Fideist". In addition, this article tries to show that it is found a sort of rationalism in Kierkegaard's works, especially when he is articulating the nature of faith; but his rationalism is not similar to the rationalism of enlightenment.