As stated in the introduction, the main objective of this thesis is to determine the feature of auxiliary verbs in modern Mongolian language and lists of auxiliary words, the meaning of each word, and its positions and roles in Mongolian language. In linguistics auxiliary verbs are rather similar to main verbs but have their own features that often attract linguistic researchers` attention. Specially, the issue about auxiliary verbs weighs mainly in debates which was related to auxiliary verbs and main verbs. Any bigger studies on auxiliary verbs in linguistics can be done through Pagva`s independent research. Therefore, we aimed for detailed studies on the feature of auxiliary verbs and tried to determine their specific meanings and importance in the general research up until now. After setting up the objective, we evaluated studies on auxiliary verbs of modern Mongolian language up to the present and made list of auxiliary verbs on a basis of own study category by using our evaluation and conclusion. We tried to unite and assess the studies on auxiliary verbs of Mongolian language. Moreover, we would like to determine our research methods and explain eight different types of features to distinguish the main verbs from auxiliary verbs. These features became the main measurement to make distinction between auxiliary verbs and main verbs as well as the main study of this thesis. We used schemes to show the positions of auxiliary verbs in sentences and explained plainly using sentence structure. We provided examples on recognizing sentences with auxiliary verbs from sentences with combined verbs. Using eight features of the auxiliary verbs as starting points, we determined the features of auxiliary verbs to differ them from other verbs and listed auxiliary verbs studied by researchers and reviewed them within our study. In addition, auxiliary verbs were categorized through a basis of their meanings and those with similar meanings were contrasted showing their similarity and dissimilarity. All the auxiliary verbs were classified depending on their linking suffixes and explained about the changes they are making if there were any.