This study explores how syntactic complexity and lexical complexity vary in argumentative essays written by L2 learners of English at B2 and C1 CEFR proficiency levels. We approach both syntactic complexity and lexical complexity from a multi-dimensional perspective, examining syntactic complexity at different levels (i.e., global, clausal, and phrasal) and including the three lexical dimensions as diversity, density, and sophistication. Undergraduate students (n=42) studying English Language Teaching at a private university in Turkey voluntarily participated in this study and completed an argumentative essay writing task. We manually coded their essays for syntactic complexity structures and used the automated analyzer developed by Lu (2012) for lexical complexity features. We calculated descriptive statistics separately for the lower-level and higher-level groups and investigated the syntactic complexity variation among the two groups using the nonparametric Mann-Whitney U test. Our results indicated significant variation among the two proficiency levels in three syntactic structures (finite complement clauses controlled by nouns, words before the main verb, and passives), while there was no significant difference between the groups in lexical complexity. These findings contribute to the understanding of the connection between linguistic features and L2 writing proficiency levels.