Abstract: This study aims to investigate, one of the most significant issues of writing skill, post-modification, often causing difficulties for EAP students struggling for making complex constructions in their academic writings. A contrastive analysis procedure is applied, which involves discovering how Post-nominal Modifications (PNMs) in English and Persian languages compare and how writers with differing language backgrounds manage to use complex constructions. Data composing of academic articles in the field of psychology written by three groups of writers were randomly selected for comparison. The quantitative analysis of data indicated that non-native writers displayed frequency differences in using PNMs compared with native academic writers. On the other hand, data examination revealed, although Iranian writers faced with difficulty in creating complex constructions, frequency studies did not show any significant differences between their writings and native writers. The explanations for this surprising finding were provided evidencing the effective role of L1 and faulty constructions were examined qualitatively demonstrating clear traces of L1 interference resulting in grammatical mistakes observed in the inter-language productions of Iranian EAP writers.