Idioms are set of words whose meanings are not easy to understand or guess from the literal usual meaning of those words. The present study is an attempt to show the relationship between Idiomatic Knowledge and Second Language Proficiency of undergraduate students of English in some Kurdistan Region/ Iraq universities. The study has been carried out at English Departments of seven randomly selected Kurdistan universities using a quantitative method to highlight the level of the students in understanding idioms and the challenges or the difficulties they face in using them. Two tests have been adopted to get the data from the departments concerned after getting official permissions. Definitions by different linguists and dictionaries with classifications of idioms are provided. Then, some detected problems have been displayed and analyzed such as avoidance, competence and culture. Finally, the study has come up with some important conclusions and suggestions among which the existence and the nature of the relation between Idiomatic Knowledge and Second Language Proficiency.