The features of national mentality are commonly emerged in its language and more specifically in its idioms and idiomatic expressions. A study on idioms plays a prominent role in the studies on Mongolian lexicology. In addition to studies clarifying and explaining the meaning of Mongolian idioms thus identifying the features of national mentality, recently a number of comparative studies on idioms of two different languages have been published. The social disintegration that happened throughout history in Mongolia caused differences in language dialects of Mongolian ethnic groups. Some Mongolian nomads had moved to the settled culture earlier than others. Therefore, the main objective of this study is to clarify the distinctive features of two dialects of the same language that are coexisting in two different countries, cultures and social relations by examining their idioms. Studies on Mongolian idioms and proverbs and comparative studies with other languages have been published in the past, however this study is the first comparative study on idioms of two dialects within the Mongolian language. This paper aims to introduce the language status of Xinjiang Oirats and to explain language evolution by examining the semantic and lexical changes in idioms of two dialects of one language.