Background: The P600 brain wave reflects syntactic processes in response to different first language (L1) syntactic violations, syntactic repair, structural reanalysis, and specific semantic components. Unlike semantic processing, aspects of the second language (L2) syntactic processing differ from the L1, particularly at lower levels of proficiency. At higher L2 proficiency, syntactic violations are more likely to result in P600, similar to the L1 native speakers. Objectives: This study aims to assess the effect of proficiency on L2 syntactic processing in late bilinguals and determine whether L1-like cerebral activation patterns will result. Materials & Methods: In this descriptive quantitative research, the subjects were two groups of Persian-English bilinguals (L1=Persian, L2=English; n=10 high-proficient, n=10 low-proficient; gender=female who started learning English as a Foreign Language (EFL) after the age of 15 through explicit instructions. Within the violation paradigm, Event-Related Potentials (ERPs) were collected from the subjects in the neurocognitive lab of Shahid Beheshti University, Iran, in 201920. The experimental trials of the ERP task included violated English regular past tense verbs. ERP components were compared with those of the L1 (components closer to P600). Results: The t-value for P600 peak latency differed significantly only for the Incorrect past tense verb (ICV) condition and only in O2 (P=0. 039463, t=2. 2205, CI: 0. 003112-0. 11249, P<0. 05) between the two groups (higher in the high proficient group). Conclusion: P600 for the high-proficient group demonstrated that L2 proficiency was a more determinant factor in L1-like cortical representation of L2 than the age of acquisition and or the type of context.