The use of relative pronouns is an intriguing topic for learners and teachers. The past two decades have witnessed exponential growth in research on the use of relative pronouns. However, little attention has been given to relativizer omission among nonnative speakers in writing. This research examined the distribution of zero relativizers and the variation patterns of relative pronoun omission in academic writing among Chinese speakers of English. The data were collected from 396 written essays in two English-as-a-medium-of-instruction courses. Adopting a variationist approach, the written essays were analyzed by the Variable Rule (VARBRUL) analysis, a multivariate logistic regression. The findings revealed that through particular essay sentence structures, syntactic functions, type of antecedents, and length of relative clauses, one could predict the writer's choice of zero relative pronouns. The reasons for variation can be explained by the effects of language processing, predictability or frequency. Some pedagogical implications are proposed.