The study explored EFL students' development of syntactic complexity by employing the Latent Class Growth Analysis (LCGA) approach. A total of 214 tertiary EFL students from Southwest China were invited to write four argumentative essays over an academic semester. The unconditional models of LCGA were utilized to explore the optimal latent classes of students' development trajectories of syntactic complexity. The conditional models of LCGA were employed to investigate the predictive effect of English proficiency on the optimal latent classes. Results of the unconditional models revealed different latent classes of development trajectories for six indices of syntactic complexity rather than the remaining ones, which offers tentative evidence for the heterogeneity of L2 development trajectories. Results of the conditional models showed that English proficiency did not predict the membership in these latent classes. These results are discussed and implications for L2 instruction are attempted.