Syntactic complexity is a key index to students’ development in second language writing. Although widely researched from students’ perspective, little is known about how teachers revise student’s writing in terms of the syntactic complexity. This study compares differences in the syntactic complexity between L2 students and a teacher. Thirty students’ argumentative essays were matched-up with a native-speaker teacher’s writing to compare syntactic complexity and the grammatical structures. Using a computerized tool, their writings were examined for complexity in five complexity dimensions and significant differences were found for 10 measures of complexity. A follow-up analysis further identifies three grammatical structures that may deserve the teacher’s attention. Results in this study demonstrate that a concrete understanding of L2 grammatical development can be gained if the teacher’s revisions, as well as students’ writing, are examined in syntactic complexity research.