In this paper I introduced some linguistic proofs that the Aztecs, Mexican indigenous people, used the Korean language. In order to prove it, I studied first correspondence of vocals and consonants between Nahuatl and Korean, and explained the importance of morpheme correspondence in comparison of Nahuatl and Korean words. I compared five Nahuatl words like macana, macahuitl, patollin, chinampa, and chinamitl with corresponding Korean words. I analysed their morphemes and compared them with respect to their pronunciation and meaning. Afterwards I compared the process of combining morphemes of each word with that of corresponding Korean morphemes. I also compared them to see if some grammatical elements like suffix are equally used, if necessary, in the same position in the process of combining morphemes in two languages. As the results of these analysis and comparison, I can get to the conclusion that those five Nahuatl words are Korean because all morphemes of each Nahuatl word and their combining processes are the same as the corresponding morphemes of Korean words and that the two languages used same grammatical elements in the same positions in combination of morphemes. It is impossible that these correspondences occur by chance. These correspondences are only owing to the fact that Korean ancestors moved to Mexico in an ancient times.