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As an element connecting the four language skills in EFL, vocabulary plays themost significant role in foreign language learning. In order to communicate wellin a foreign language, students should acquire an adequate number of words andshould know how to use them accurately and appropriately. However, there arestill many of students who lack of vocabulary. One of the ways to improvestudents’ vocabulary mastery is through Extensive Reading. Extensive Reading ispurposely focused on students’ reading as much as possible: not only in classroombut also out of classroom, as cited in Day and Bamford (1998, p. 4).This research aims to find out how Extensive Reading improves students’vocabulary mastery at grade 10 of SMA N 1 SEWON. The research question ofthis study is how does Extensive Reading improve students’ vocabulary mastery? This research conducted Classroom Action Research. The subjects of thestudy were 33 students of X MIPA class 2 of SMA N 1 SEWON. In collecting thedata, the researcher used some instruments, namely; pre-test, progress test, post-test, field notes and interview guidelines questions. The data of this study werequalitative and quantitative. Qualitative data were obtained from the field notesand the interview. Quantitative data were obtained from pre-test, progress test andpost-test. The implementation of Extensive Reading in this research is by askingthe students to read English story for 10-15 minutes in every meeting.The results showed that Extensive Reading can improve students’ vocabularymastery based on the increasing in students’ vocabulary mastery and the changesin students’ behavior. It was found out by measuring the mean score of the testsresult. The improvement was also shown from the reflection of Cycle 1 and Cycle2. It is shown that the students know much new vocabulary. In addition, theactivities made the students more enthusiastic in teaching and learning process.

vocabulary mastery, extensive reading, classroom action research (CAR)