This article follows two purposes or main targets: the first is the mutual relationship between peace and happiness, and the second is the endless interaction and opposition between them. Obviously, instead of a descriptive approach, the writer follows an analytic or descriptive-analytic method in exploring the main point of the paper. The innovativeness of the writer here lies in his discovering the developmental mutual relationship between peace and happiness within the lines of Mulla Sadra’s works and explaining it accordingly. This relationship is analyzed based on various principles in his philosophy. According to the different angles of Mulla Sadra’s anthropology, peace is the necessary condition for worldly and otherworldly happiness. At the same time, happiness is the necessary condition for consolidating peace. The Sadrian ontology and cosmology are the bases of the writer’s analyses regarding the above-mentioned mutual relationship. Here, he explicates how the Sadrian Society, while enjoying plurality, can be profoundly monistic, as well. An analysis of the relationship between peace and happiness based on this philosopher’s religious ideas illustrates how severely peace is endangered in the modern world. The structure of the government and its functions unveil this mutual relationship in a different sense, in the same way that Mulla Sadra’s ethical ideas reveal the exact relationship among peace, security, and moral happiness.