He was the son of King Naemul and Lady Boban, who was the daughter of King Michu.
Nulji married the daughter of King Silseong of Silla, who nonetheless exiled Nulji's younger brothers as hostages to Goguryeo of northern Korea and Wa of Japan. Silseong also plotted to have Nulji killed, but with Goguryeo aid, Nulji was able to kill Silseong in 417, after which he ascended to the throne.