Zhi-yi wrote Tendai-sandai-bu (three major works on the Lotus Sutra, or three major writings of the T'ient'ai school) of "Hokke gengi" (Essentials of the Lotus Sutra) and "Hokke Mongu" (Words and Phrases of the Lotus Sutra) and "Makashikan" (Mahayana Practice of Cessation and Contemplation) and made Hokekyo Sutra Konpon kyoten and taught Kyoso Hanjaku (Formation theory of sacred books) of Buddhist Doctrines known as "Fivefold Periods and Eight Teachings" (among the Buddhist scriptures which were written according to the five levels of understanding on Buddhism, Hokekyo Sutra was the pinnacle).
智顗は『法華玄義』『法華文句』『摩訶止観』の天台三大部を著して、『法華経』を根本経典とし、五時八教（仏教の理解度の5段階に合わせて記された経典のうち、法華経を到達点とする）の教相判釈（経典成立論）を説く。