The purpose of this study is to analyze the grammatical complexity features of international teaching assistants' (ITAs) mock-teaching presentations and to compare the distributions of these features to those found in the Oral English Proficiency Test (a local ITA assessment), university classroom teaching, conversation, and academic writing. The data consisted of 186 prospective ITAs' mock-teaching presentations collected from two ITA training courses at a large U.S. university: one higher-level course and one lower-level course. All presentations were transcribed, proofread, and built into a corpus consisting of 247,043 words. Based on the previous corpus studies, a total of 22 grammatical complexity features were selected for the analysis. Overall, the discourse of ITA mock-teaching involved a substantial number of both the grammatical complexity features commonly found in spoken conversation (e.g., finite adverbial clauses and modals) and those commonly used in academic writing (e.g., noun premodifiers and prepositional phrases). The frequency distributions of the features differed in complex ways between ITA mock-teaching and the other registers, as well as between the higher-level and lower-level ITAs. Pedagogical implications for ITA training and assessment, and future research directions are discussed at the end.