In 738, Niu was made "Shizhong" (), the head of the examination bureau (門下省, "Menxia Sheng"), a post considered one for a chancellor; he was also made the deputy jiedushi of Hedong Circuit (河東, headquartered in modern Taiyuan, Shanxi), but remained at the capital Chang'an to serve as chancellor. He was further given the additional post of minister of defense in 739 and was responsible for selecting military officers, just as Li Linfu was made the minister of civil service affairs and was responsible for selecting officials. In 740, he was stripped of his commands of Shuofang and Hedong, but remained chancellor.