The title of this thesis is “Four dhyānas in Zhiyi’s Cessation-Contemplation System”, and the main purpose to discuss the four dhyānas expounded by Zhiyi, which included its theoretical basis and characteristics. In addition, I also tried to discuss four dhyānas in the practice of perfect-sudden meditation. First of all, in terms of the definition meaning, the four dhyānas discussed by Zhiyi, illustrate the expansion of theoretical annotation from the original stereotypes. The explaination of the attainment in the first dhyāna and dhyāna factors (禪支) is expounded through eight sensations (八觸). Regarding the second, third, and fourth dhyānas, besides quoting from the viewpoints of the sutras and treatises, and also included some of Zhiyi’s own interpretation. From the structure and content of the four dhyānas in the “Shi chanboluomi cidifamen” (《釋禪波羅蜜次第法門》), the main objective is to practice and achieve the four dhyānas. In particular, before the first dhyāna, Zhiyi establish the four prestages which are cuzhu (麁住), xizhu (細住), kāma-dhātu-samādhi*(欲界定), anāgamya-samādhi (未到地定). These is one of the distinguish features of four dhyānas in T’ien-t’ai school. From the prespective of perfect teaching in T’ien-tai, the four dhyānas itself is intrinsically identical to “middle truth” (中道實相) and also equivalent to all practice (一切行). The preparation stages and the four samādhis in the perfect-sudden meditation, which illustrates the present from the basis of the four dhyānas. When four dhyānas arise during the practise, it will become the target of contemplation in order to realise the ultimate truth.