The Ara Pacis is made up of the actual altar table, on which animal carcasses and wine were offered to the gods, and the enclosure which surrounded it. The altar table takes up almost all of the internal space of the enclosure; between the two there is only a narrow passageway, with a pavement sloping gently towards the exterior, in such a way as to allow water, both rain and water used in the ritual of sacrifice, to flow out of the structure, through small drainage canals along the perimeter.
The altar stands on a podium of four steps, on top of which is the altar base, with another four steps at the front only. Above this rises the altar table, closed between a shelf on each side. The two sides supports are decorated with acroteria, floral volutes and winged lions.
On the inside of the left side wall the Vestals can be seen, six in all, shown with their heads covered: these were the girls appointed by the Pontifex Maximus, the high priest of the city, chosen from the daughters of the aristocracy between the ages of six and ten, who then remained the guardians of the sacred flame for 30 years. Here they are shown, accompanied by their assistants, as the ceremony takes place.
Only a fragment remains of the frieze opposite that of the Vestals. It shows two figures, the first of whom is a priest, or more precisely a flamen, while the figure following him is sometimes identified as Augustus himself, possibly shown wearing the attire of the Pontifex Maximus, a position which he assumed in 12 B.C. while the Ara Pacis was being built. On the outside of the right support a procession of three animals has survived, two cows and a sheep being led to the sacrifice by twelve adepts ( victimarii). In their hands they hold the sacrificial instruments: trays, the knife, the club, the laurel branch for aspersion. They are proceeded by a man wearing a toga (perhaps a priest) accompanied by helpers and acolytes of the cult. Most probably the fragments of the altar frieze refer to a particular sacrifice, perhaps that of the Pax Augusta itself, which the Senate had decreed should be celebrated every year on the 30th of January, commemorating the consecration of the altar.