In second language (L2) studies, one direction of research is to investigate whether adult L2 learners still have access to Universal Grammar (UG) .In the related vein, this study is an indirect assessment of availability of Wh-constraints in interlanguage grammar of Persian EFL learners through an on-line sentence-matching task. To do so, 60 university students, both male and female, majoring in EFL at Khorasgan university in Iran were chosen through an Oxford Placement Test (OPT) and divided into three proficiency groups(i.e. 20 Low Intermediate , 20 High Intermediate, and 20 Advanced groups). Moreover, 10 native English speakers were chosen as the control group in this study. Both English native speakers and Persian EFL learners performed an on-line sentence-matching task .The on-line test contained 40 pairs of English sentences, 20 matching grammatical pairs which observed the UG Wh-constraint and 20 matching ungrammatical pairs which violated this Principle. In sentencematching task students responded to two sentences on a computer screen indicating whether the two sentences were identical or not. The reaction time of EFL learners to both grammatical and ungrammatical pairs was measured and compared to that of English native speakers. The results revealed that native speakers of English can do the task faster than non-natives. Thus, Whconstraint is not accessible to second language learners to the same extent that it is to first language learners. This study furthers our understanding of the Persian EFL learners' performance in the area of L2 language acquisition. Moreover, the more proficient groups were faster than the less ones in sentence-matching task. As the proficiency increased, the results got much closer to the results of native speakers . Therefore, proficiency can be considered as a strong factor for UG activation.