The purpose of this study is to review the accomplishments of Eon'gan study in the Korean Language History based on Eon'gan data of Joseon Dynasty and to predict future studies. The main reasons of gathering interest in Eon'gan data are the excavation of family letters such as < Suncheo'ngimss'iEongan > and < Jinjuhass'iEongan > and the publication of translated books of Eon'gan. Furthermore, the interpretation book of Eon'gan has played a key role in enhancing the reliability of Eon'gan data. Regarding writing and phonographical matters, palatalization and liquid consonant along with ‘ㆍ’, ‘ㅸ’ and ‘ㅿ’ were main interest. The most active discussion among them is the phenomenon of palatalization, and it is noticeable that not only was the practice of palatalization discussed but also the change by various factors of sociolinguistics was discussed. The grammar part is the most actively discussed among Eon'gan studies. Especially, such matters as nominative marker and relative honorific expressions can be considered as the most meaningful accomplishments in Eon'gan data. In addition, there is active discussion on pronouns, individual ending words and individual morpheme. Studies on vocabulary based on Eon'gan data show high interest in Eon'gan's own vocabularies that do not appear in printed books. Recently, names of articles, disease related words, unit nouns and names of places are also objects of main interest. Meanwhile, the number of Eon'gan data introduced to the academic world is 3,296, and there is a need for expansion of Eon'gan data through deep learning. There should be active sociolinguistic discussion that can highlight the strengths of Eon'gan data while the necessity of computational linguistics for establishment of corpus is discussed. The difference between males and females can be a future research subject. It is necessary to elaboratize the details of the existing Korean language history through Eon'gan data, along with the description of tentatively named "Korean language history of Eon'gan'.