Since modal auxiliary verbs have been proved to be one of the most troublesome grammatical structures in English, the researchers of this study decided to do an analysis on the ways in which advanced EFL Iranian students use modal auxiliaries focusing specially on nine modals' semantic functions. Consequently, was conducted based on the following object: To investigate the semantic functions depicted by modals used by advanced adult EFL learners of English. To do this, a learner-corpus was constructed with 136 compositions written by our learners and then Wordsmith Tool was used to analyze this computerized data. The results of this study show that some meanings are overly used (like "ability" meaning of can) and some of them are not really used by our learners like the "possibility" meanings of can and could. At the end, some pedagogical suggestions have been made to improve this situation.