By examining the rhyming of Han Yu’s Old Style Poems, Ciri Zuke Xi and Yuanhe Shengde Shi, this paper argues that Han Yu was aware of the differences of rhyming in Archaic and Ancient Chinese. He also attempted to imitate the rhyming as seen in the Archaic Chinese corpus, but his imitation was problematic due to the inadequacy of his knowledge of Archaic Chinese Phonology. Regarding those of Han Yu's rhymes that are not completely accurate, Ouyang Xiu analyzed these rhymes from the perspective of Pangyun (Lateral Rhyme), while Wu Yu, Zhu Xi and Yan Yu introduced the concept of Guyun (Old Rhyme). The investigation of Guyun was highly relevant to the Neo- Confucianism of Northern Fujian and the popularity of Han Yu's work in Fujian. From Wu Yu's Yunbu to Yan Yu's Canglang Shihua, Han Yu the "Zixue (Studies of words) expert" was always the key topic of Guyun studies. This paper also states the similarities between Guyun and the Min dialect in the Song Dynasty, and comes to the conclusion that such similarities contributed to the Neo-Confucianists' sensitivity towards Guyun. Their choice of Jiyun as the reference of Guyun illustrates the conservative attitudes of Min Neo-Confucianists.