This thesis is aimed to (1) identify how corpus is used to improve students’ vocabulary mastery at fifth grade students of SD Ta’mirul Islam Surakarta; and (2) identify the strengths and weaknesses of corpus when it is implemented in the teaching learning process of English.

This classroom action research was conducted at SD Ta’mirul Islam Surakarta in the Academic Year of 2014-2015 while the subject of this research is fifth grade students of it. This research was carried out in two cycles. Each cycle consisted of four steps: planning, acting, observing, and reflecting. The qualitative data were collected by using interview, observation, and questionnaire while quantitative data were collected by using tests (pre-test and post-test). There were five steps in analyzing qualitative data: assembling the data, coding the data, comparing the data, building interpretations and reporting the outcomes. Then, quantitative data were analyzed through descriptive statistics.

The research findings show that corpus can improve students’ vocabulary mastery. There were three indicators which pass the passing grade: (1) identifying the meaning of the words (79.58); (2) pronouncing the words (76.67); and (3) spelling the words (76.25). One indicator didn’t pass the passing grade: using the words in a sentence (61.67). Teaching vocabulary using corpus also has strengths and weaknesses. The strengths are: (1) corpus could attract students’ attention to the material; (2) the students were active to participate in teaching and learning process; and (3) the students had a bravery to share their idea among the students while the weaknesses are: (1) the use of corpus needed more time allotment; and (2) the students got bored when they analyzed corpus in long time.

Realizing the usefulness of corpus to teach students vocabulary, the teachers or students can use it as one of alternative media to teach or learn English especially vocabulary.

Key words: classroom action research, vocabulary mastery, corpus