2Instructor, Parssabad University of Applied Sciences and Technology, Ardebil
Reading comprehension plays a central role in nonfiction general English university courses in different EFL situations and one of the important factors affecting it is vocabulary size. Accordingly, the present study aimed at investigating the effect of unknown vocabulary size on EFL learners’ reading comprehension. To this end, 40 TEFL freshmen were ed to participate in this study on the basis of their scores on Nation’s Vocabulary Levels Test (VLT) (2001). They were then given five versions of the same text with different densities of unknown vocabulary, i.e. 80%, 90%, 95%, 98%, and one intact version. A one-way ANOVA and a regression analysis were used to analyze the obtained data. The results showed that different densities of unknown academic vocabulary resulted in significant differences in comprehension of nonfiction general English texts and there was a vocabulary coverage level which acted as a threshold between adequate and inadequate comprehension of these texts. The findings are interpreted to have implications for EFL teachers and university instructors as well as syllabus designers and materials developers. The significance of the present study lies in the fact that unlike many previous studies it is conducted on nonfiction texts, namely nonfiction general English texts.