The present study replicated the research framework of a previous study (Biber, Gray, & Poonpon, 2011) that identifies the grammatical complexity of L1 professional academic prose as strongly favoring a dense use of phrasal nominal modifiers such as prepositional phrases as postmodifiers, attributive adjectives, and nouns as premodifiers which characterize its unique structurally compressed discourse style. The main purpose of the present study was to explore syntactic similarities and differences between L1 professional and L2 student academic writing in terms of their reliance on phrasal/nominal compression features to determine characteristics of the grammatical complexity of advanced ESL academic writing. To this end, the distributional patterns of use for 25 specific grammatical complexity features of structural elaboration and compression were investigated in a corpus of 128 short academic essays collected from 16 advanced ESL learners and 16 L1 university students (as comparison data).The results showed a heavier reliance of both the advanced ESL and L1 student academic writing on phrasal nominal modifiers (attributive adjectives and prepositional phrases as postmodifiers) of structural compression than on clausal elaboration features, which lent empirical support to Biber, Gray, and Poonpon’s (2011) findings. In addition to the phrasal compression features, both the advanced ESL and L1 student academic writing were also characterized by a prominent use of specific colloquial grammatical devices such as adverbs as adverbials. Compared to the advanced ESL writing, the L1 student academic writing showed a significantly more preference for one particular colloquial feature: ZERO relative clauses where relative pronouns replacing relativized objects are omitted. This combined reliance on both phrasal compression devices and colloquial features in both the advanced ESL and L1 student academic writing distinguished their grammatical complexities from that of L1 professional academic prose and signaled a possibility for recognizing them as a transitional developmental stage from more casual to more academic writing.