This paper is to examine Chungbuk dialects of the 19th to 20th century shown in Korean old vernacular letters of Song Byeong-Pil Families, especially focusing on the phonology of vowels. Its purpose is to investigate the process of changes in the vowel phonology shown in Chungbuk dialects of this period. The total number of Korean old vernacular letters of Song Byeong-pil Families addresses in this study is 91, written by the generation of Song Byeong-pil and previous(-1) and next(+1) generations. They could be roughly divided into three generations, roughly by a sender, and letters from Song Ji-soo, of the first generation, Song Byeong-pil, of the second generation, and Eunjin Songssi of the third generation are precious records showing changes made to a language. The most significant phonemic phenomenon shown in these letters is vowel raising, especially vowel raising of ‘어>으(ə>ɨ)’. It was incurred in the late 19th , the generation of Song Byeong-pil and it is assumed that at the early stage of this transition, this vowel rising was also made on a word-internal position even for non-anlauts, not just on a long vowel of an anlaut. In addition, vowel raising of o>u seems to started in the generation of Song Byeong-pil and it was shown on a few lexical morphemes, but not grammatical morphemes. On the other hand, vowel raising of e>i was shown in very limited cases and also in Korean old vernacular letters of Eunjin Songssi. Therefore, it could assume that this type of vowel raising was incurred the latest. In these records, the transition of ‘ㆍ>ㅓ’(ʌ>ə) was confirmed in only a few letters. However, these types of transition were not more frequently used in following generations. Instead, they were more shown in letters of Song Ji-soo and Song Byeong-pil than of Eunjin Songssi. These trends could be interpreted as that the transition of ‘ㆍ’ on non-anlauts was started before and confusion was continued during the period and then, started to be stabilized as ㅡ’ or ‘ㅓ’. Naegative vowelization among vowel harmony of ʌ/ə was also confirmed and ‘몰너(molleo), 바더(badeo), 안져(anjyeo), 아러(areo), 차져(chajyeo)’ and others could be examples of naegative vowelization. These examples are shown in letters of other family, such as of Lee Young-yeon, of the first generation. This fact could be an evidence of that transition to negative vowelization was started in other regions and then, expanded to this area.