This paper theoretically and empirically addresses the issue of measuring the grammatical complexity of English language learners’ writings. For testing the validity of certain measuring indices, the authors gauged the sample articles’ T-units, dependent clauses, non-finite verbs and connectors based on Chinese Learners’ English Corpus and found that these indices are valid in measuring second language learners’ writing grammatical complexity. Meanwhile, T-units, non-finite verbs and connectors may well predict the development of the grammatical complexity of the second language learners. The study also discovered that non-finite verb is a trustworthy index in measuring second language learners’ grammatical complexity in writing for its better performance in the statistical tests of this study. The results indicate that the non-finite verb is a special index in measuring learners’ language complexity while dependent clauses contribute less significantly to the development of learners’ grammatical complexity.