At the end of the Tang period, Confucianism, Buddhism, and Daoism were active as the Three Faiths. Daoism had been adopted as the state religion, and Buddhist and Confucian believers were in decline. Groups aiming to revive Confucianism arose in the bureaucracy, and drew close to Buddhism. Li Ao 李翺 is known as a person who blended Buddhism and Confucianism. However, his teacher, Han Yu 韓愈, was at the forefront of opposition to Buddhism. Why did Li Ao introduce Buddhism into Confucianism? In this paper, analyzing the Fuxing shu 復性書, an important work of Li Ao, I compare it with the thought of Han Yu.