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In order to establish the why of noticing, it is imperative to empirically explore the factors that potentially mediate noticing. This study aimed to explore two factors that are believed to affect noticing:  the complexity of target structures and learners’ second language (L2) proficiency level. English relative clauses (RCs) were selected as the target structures, and Accessibility Hierarchy Hypothesis (AHH) was taken as the measure of the complexity of the RCs. A sample of 113 freshmen English language majors were selected as the participants of the study. A test of English RCs was developed as the placement test, and Key English Test (KET) was used to classify the participants into three groups of High (N=38), Mid (N=37), and Low (N=38) L2 proficiency level. Note-taking was used as the measure of noticing. After administering the RC test and the proficiency test, the participants were given a number of authentic reading texts containing instances of RCs and were required to take notes during reading activity. The non-parametric Friedman’s test demonstrated that the complexity of RCs positively affected the participants’ noticing while the non-parametric ANCOVA indicated that the participants’ L2 proficiency level had no significant effect on noticing. The findings of this study can be helpful to both teachers and material developers in providing learners with optimal conditions for noticing linguistic forms, which in turn, could facilitate L2 learning.