Expanding the English vocabulary for Iraqi second language (L2) learners is a challenging task. In the present work, Langacker’s cognitive linguistic theory of domains (1987) is adopted to provide a new sight into vocabulary learning, and to expand the vocabulary repertoire of Iraqi students. This paper aims at testing the validity of expanding the English vocabulary of L2 Iraqi learners throughout the theory of domains. It also tries to find how can domains theory increase the package of vocabulary for L2 learners. Accordingly, an experimental study is to be conducted on forty-six college students of second-year level from, University of Baghdad, Iraq. The data of the pre-test and post-test were analyzed using the Statistical Package for Social Sciences (SPSS) l editor. Results have shown the following: First, the progress of more than (0.05≤) has been detected as far as students' understanding of the semantic domains of the lexical concepts. Second, the main source of difficulty regarding vocabulary acquisition has been treated throughout Langacker's theory. Third, the domains theory has proven its effectiveness in accurately comprehending the semantic domains of the English lexical concepts.