The current study presents an instrumental phonetic analysis of Korean monophthong vowels in the early twentieth century Seoul Korean, based on audio recordings of elementary school textbooks Botonghakgyo Joseoneodokbon (Korean Reading Textbook for Elementary School). The data examined in this study were a list of the Korean mono syllables (Banjeol), and a short passage, recorded by one 41-year-old male speaker in 1935, as well as a short passage recorded by one 11-year-old male speaker in 1935. The Korean monophthongs were examined in terms of acoustic analysis of the vowel formants (F1, F2) and compared to those recorded by 18 male speakers of Seoul Korean in 2013. The results show that in 1935, 1) /e/ and /?/ were clearly separated in the vowel space; 2) /o/ and /u/ were also clearly separated without any overlapping values; 3) some tokens of /y/ and /ø/ were produced as monophthongs, not as diphthongs. Based on the results, we can observe the historical change of the Korean vowels over 80-90 years such as 1) /e/ and /?/ have been merged; and 2) /o/ has been raised and overlapped with /u/.