The Old Uighur individual letters were among the manuscripts found in the Thousand Buddha Caves in Dunhuang. It is seen that the Old Uighur letters identified were written on the issues of human relations, work life, state administration, trade, agriculture and daily life. There are more than two hundred letters from the Buddhist, Manichaean and Christian surroundings. This study consists of reviewing and presenting the two titles in the doctoral dissertation we prepared on Old Uighur letters and announcing the updated results of the this part of the research. The vocabulary of the letters was compared with the vocabulary of Khakas through related studies and dictionaries and traceable (common) words were determined. In addition, the common Old Uighur and Khakassian words were evaluated according to Morris Swadesh's final 100-words list published in 1971. We can explain the reasons for this comparison by the fact that Khakas, which is in the azaq / taγlïγ group of Turkish dialects, together with Yellow Uighur and Fu-yu Kyrgyz, is one of the closest contemporary Turkish dialects to Old Uighur today, after Yellow Uighur, which is not a written language, and by requirement to describe this aspect of Old Uighur-Khakas language relations.