This experimental study investigates Persian learners’ clustering acquisition of the properties associated with non-pro-drop parameter in English as L2. Research has indicated that there are certain correlations between superficially unrelated linguistic phenomena in L1 acquisition that are interpreted as the clustering effects of one on the others within the Principles and Parameters framework. For instance, in German L1 acquisition, there is evidence for a clustering acquisition of subject-verb agreement and the decrease of (incorrect) null subjects (Clahsen, 1995; Clahsen et al, 2004). Likewise, there are general arguments that the same processing mechanism happens in L2 acquisition. Thus, the present research will report the results of a Grammaticality Judgment Task (GJT) investigating the clustering appearance of L2 syntactic features related to non-pro-dropping parameter; such as obligatory overt subjects, V-S constraint and that-trace filter; in 60 Persian learners of English divided into two proficiency levels. Furthermore, each level consists of two different kinds of learners in term of the start age of L2 acquisition. They are referred to as early-starters who started L2 acquisition at the age of 7-8 and late-starters who first attended English classes at age 12-13. Our findings revealed that the three syntactic phenomena related to non-pro-dropping parameter cluster almost perfectly in post-intermediate learners indicating that proficiency level has a major influence on how bilinguals process L2 linguistic input. Moreover, we observed no difference between the performance of early and late starters with respect to clustering emergence of the above syntactic variables.