The purpose of this study is to investigate conventional expressions found in Eongan rule book during the Joseon Dynasty period. To do so, first, it discussed the defintion of conventional expressions and how they are specifically used in Eongan. Especially, to investigate conventional expressions in Eongan, this study reviewed conventional expressions presented in Jigbo-eon-gan-dok, which is one of the most well-known Eongan rule books, and how they have been applied to Eongans that have been preserved so far. Specifically, words that were frequently used in 36 letters of greetings in Jingbo-eon-gan-dok were extracted and classified according to the meaning and syntactic structure. The words frequently mentioned in Jingbo-eon-gan-dok, such as ‘bokmo,’ ‘hyangnyeom,’ ‘geulsi,’ ‘geulwol,’ and ‘haseo,’ are mainly found at the begnning and ending of the letters. Other frequently used words include ‘giwun,’ ‘geuihu,’ ‘geuitaehu,’ ‘chyongchyong/chongchong,’ and ‘chongyo/chyongyo.’ Among these words, the vocabulary that is appropriate to use to seniors includes ‘bokmo,’ ‘geulwol,’ ‘haseo,’ ‘geuihu,’ and ‘geuitaehu,’ which can be classified as an [+honorific] feature. Meaningful structures of conventional expressions found in Jingbo-eon-gan-dok are such as ‘bokmo buridi mot-,’ ‘hyangnyeom ganjyeolha-,’ ‘geuityehu mananha,’ ‘chongyo iman geuichi,’ and ‘chongchong iman jeok.’ Conventional expressions used in Jingbo-eon-gan-dok began to be found in actual Eongans from the 17th century before they became more widely adopted from the mid-18th century and most commonly in the mid-19th century. Conventional expressions were applied in Eongans as a way to express social courtesy toward the recipient and also for convenience in writing the letters.