In this paper we investigated the properties of auxiliary verb constructions in Korean and Khalkha Mongolian. And we looked at the similarities and differences between auxiliary verb constructions in the two languages. The results of this investigation will shed light not only on the properties of the auxiliary verb constructions in the two languages, but also on their genealogical and typological relationship. Most of the auxilianes in both Korean and Khalkha Mongolian are versatile in that they can be used both as auxiliaries and main verbs. In their auxiliary use, the meaning of main verbs are bleached. For example, peli- in Korean and orxiin Khalkha Mongolian mean ``to throw away`` when used as a main verb. However, as auxiliaries, they express completive aspect. Korean and Khalkha Mongolian have many connective endings but only a few of them are used in auxiliary verb constructions: -e, -ko, -ci, -key in Korean and -j, -aad, -n, -saar in Khalkha Mongolian. The endings -ko, -e in Korean and their rough equivalents -j, -aad in Khalkha Mongolian are most frequently used in auxiliary verb constructions. Auxiliary verb constructions in both Korean and Khalkha Mongolian are used to express a variety of grammatical functions, such as aspect and modality. In both languages, auxiliary verb constructions may express aspectual distinctions such as completive, progressive, and inchoative aspect. Modal distinctions such as the benefactive, trial, permission are also expressed through auxiliary verb constructions. While benefactive modality is expressed by the auxiliary verb construction -e cwu- ``-Conn give`` in Korean, Khalkha Mongolian distinguishes two types of benefactive constructions depending on whether the favor is conferred on the referent of the subject (-j av- ``-Conn take``), or that of the non-subject (-j og? ``-Conn give``). Another interesting difference between the two languages is lexical restrictions on auxiliary verb constructions of completive aspect. The -e iss- ``to be, exist`` construction in Korean imposes restrictions on verbs that can occur as main verbs, while the corresponding construction in Khalkha Mongolian, -aad bai- ``to be, exist``, does not. The situation in Khalkha Mongolian is similar to that found in Middle Korean. The process of grammaticalization is observed in both languages. Many auxiliary verbs in Khalkha Mongolian developed through the grammaticalization of lexical verbs. It is likely that auxiliary verb constructions in both languages will undergo further grammaticalization in the future.