This study modeled the effects of essay length and language features on the rated quality of second language (L2) expository and argumentative essays composed by Chinese university students. Latent variables were writing quality captured by essay scores, and lexical sophistication, syntactic complexity and cohesion, each of which was measured by different linguistic features in the essays. Results showed that: (1) essay length, sophisticated words (i.e., words eliciting longer lexical decision times, and used in academic texts), complex nominals and coordinate phrases could account for approximately 45% of the variance in the scores of argumentative essays, and (2) essay length, sophisticated words (i.e., academic words, words used in restricted context, and words that have fewer orthographic neighbors), complex nominals, coordinate phrases and word type-token ratio could explain approximately 49% of the variance in the scores of expository essays. Such findings indicate that, although differing in genre, raters tended to give higher scores to the essays of two genres that contained more sophisticated words, complex phrases, and distinct types of words.