The concept of eusebeia (piety) well known in the Greek world serves as the foundational virtue in Philo’s ethical system. He appropriates the Greek understanding of eusebeia and applies it to his ethical teaching about how one is to live virtuously. However, instead of viewing eusebeia as a subordinate virtue, like in the Stoic and Platonic traditions, Philo gives the virtue eusebeia a primary place and names it “the highest and greatest of virtues” (Abr. 60) and “the origin of all the other virtues” (Decal. 52). What is significant about Philo’s ethical system is that he subsumes all the ethical commandments of the Decalogue under this greatest virtue, eusebeia, but without divorcing himself from his Greek philosophical tradition. This paper will show how this is so. Ancient sources evince that the early understanding of the Greek concept of eusebeia went through a development (ca. fifth century B.C.E.). This paper will show how this development of eusebeia in the Greek tradition is reflected in one aspect of Philo’s use of eusebeia: that is, eusebeia for the service of God and human beings. We will see that in Philo’s ethical discussion especially concerning the Decalogue, he views eusebeia in two ways: prior to its transition, when eusebeia was used indistinctly from dikaiosune; and after its transition, when eusebeia was sharply distinguished from dikaiosune. I will also offer a reason why both traditions are still present. This discussion will not only illumine some of the complexities of Philo’s understanding of the concept of eusebeia as the primary virtue in his ethical system, but will show the influence of the Greek philosophical tradition on his thought.