The main slogan of Stoic philosophy is “Life in Harmony with Nature”, which also signifies the unity between physics and ethics in stoics’ ideas. The study of the roots of this slogan in the view of Stoic philosophers; from Zeno of Cilium, the founder of Stoic philosophy to Marcus, Aurelius, the last Stoic philosophers, indicates the expansion of the semantic domain of “Life in Harmony with Nature”, as follows: 1) individual nature in the sense of harmony with the rational faculty; 2) general nature in the sense of harmony with fate and those affairs which are beyond our control, and 3) social nature in the sense of harmony with society and social laws. Interestingly enough, in Stoic ethics there are some terms for each of these semantic domains in relation to natural sciences; for example, self-preservation, kindness, common sense, and providence in the world. This paper mainly focuses on the problem of functions of “Life in Harmony with Nature” in Stoic ethics based on the ideas of Seneca, Epictetus, and Aurelius as recorded in their existing works. Finally, it concludes that Stoic philosophers, particularly the three mentioned above, mainly emphasize the Stoic concept of “Life in Harmony with Nature” in order to create a unity between Man’s inner order and the general order of nature and society. This is because, in this school of philosophy, Man is a part of the whole and must use this relationship in order to attain happiness and harmony between themselves, society, and the whole.