This study investigated syntactic complexity of argumentative essays written by Korean university students with three English proficiency levels-beginning, intermediate, and upper-intermediate-in an English as a foreign language (EFL) context. Drawing on a corpus of 128 timed essays and analyzing them using the L2 Syntactic Complexity Analyzer (Lu, 2010), this study examined 12 indices representing four dimensions of syntactic complexity: length-based measures, subordination, coordination, and phrasal elaboration. Findings revealed a general upward trend in syntactic complexity with increasing proficiency, particularly in mean length of T-unit (MLT), dependent clauses per T-unit (DC/T), and complex nominals per T-unit (CN/T). Subordination showed a modest increase at the intermediate level and continued to increase through the upperintermediate level, where it significantly distinguished learners from beginning-level peers. Phrasal elaboration became more prominent at the upper-intermediate level.
However, coordination measures remained stable across groups, possibly reflecting L1 influence. While results suggested a developmental shift from clause-based to phrasebased structures, the lack of change in coordination suggested that this shift might not apply uniformly across syntactic dimensions. This study concludes with pedagogical implications for EFL writing instruction, emphasizing the need for targeted support in developing phrasal complexity and understanding learners’ syntactic growth.