This true longitudinal study explored the lexical development of L2 university English major students in Thailand using two learner corpora. This study analyzed the natural learners' English language writing assignments from two writing courses and tracked their lexical development over a relatively long period (three years with five batches of students). The data included four academic writing assignments for the first learner corpus and a research project for the second corpus. The two corpora comprised 619 pieces of writing with 462,842 tokens in total. The results from the VocabProfile program show that the students used academic words at a high level in their research project, revealing that they tended to develop their lexical competence after being exposed to more advanced English reading sources. The findings also suggest the effectiveness of the course management of the B.A. Program in English, which helps students improve their lexical competence to achieve the requirements for high-quality journals.