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  • This study aims to investigate the lexical complexity index in third semester English department students’ argumentative essay from week four to week twelve during the Argumentative Writing course. The data was collected from 2 classes out of 6 classes in which 44 argumentative essays were retrieved each in week four and week twelve. This study used corpus-based method as part of quantitative approach. The data were measured using three types of lexical complexity measurements in accord with Laufer and Nation (1995) which are Lexical Density, Lexical Sophistication, and Lexical Variation. The results shows that Lexical Density of the students in both class is high (0.58) but do not experience development throughout eight weeks. The Lexical Sophistication of both class experience development in which the GSL index increase by (-0.45) and (0.65) with the AWL index increase by (-0.98) and (0.31). The Lexical Variation do not experience development in which the index achieved is (0.15). In conclusion, during eight weeks of the course, there is development in the lexical complexity although the development do not occur in all the aspects.