Abstract: The subject of this research is the Yakut-Mongolian parallels of dialectic lexis of the Yakut language encountered only in the local dialects and unknown in the literary Yakut language. The object of this research is the phonetic processes in Mongolian parallels of dialectical lexis of the Yakut language. The author meticulously examines the reflexes of the Middle Mongolian affricate *č in Mongolian parallels of dialectical lexis of the Yakut language, which data corresponds with the various development stages of the old written Mongolian language. Analysis is conducted with consideration of phonetic changes that have occurred in the history of development of the Yakut and Mongolian languages. The author applies the method of comparative analysis of orthogram of the word in the old Mongolian writing with the data of dialectical lexis of the Yakut language. It is revealed that dialectical lexis of the Yakut language demonstrates the state of Mongolian dialects of the X-XIV centuries, i.e. the time when in Mongolian languages was present the indigenous *č + vowel and či taking roots in the historic *ti. The author also determines the instances, which reflect the evidence of the ancient and possibly late Yakut-Mongolian connection.