In this paper we will discuss Mulla Sadra’s rational description of happiness. Existence is a reality having analogicity and comprising different degrees for Mulla Sadra. He regards existence and perception of existence as goodness and happiness. So goodness and happiness are also realities having analogicity and comprising different degrees for him. Mulla Sadra insists that perception of existence and the union of the rational soul with the active reason make human happiness.Although his view is based on his own philosophical principles, we can find many similarities between his viewpoint and what is discussed on this subject in ethics and religious texts.