This study aims to determine the improve in students' English vocabulary mastery using crossword puzzles and to find out what is the effect of using crossword puzzles in the learning process. This study uses a class action research method. Researchers conducted research at a junior high school in the city of Pontianak and the subjects in this study consisted of 36 students. For data collection, researchers used observation checklists, field notes and vocabulary tests, to find out whether there was progress from students in improving their English vocabulary mastery.
This research was conducted in two cycles. In the first cycle the average value of students is 77.22 and is categorized as low or lacking. Then the researcher continued to the second cycle by using classroom action research steps and the researcher focused more on attracting students' attention to focus on learning and explaining instructions for working on crossword puzzles. Therefore in cycle II the average value of students increased to 80.69 and in the good category.
This shows an improve in mastery of English vocabulary after implementing crossword games in the learning process in class. The results of this study concluded that there was an improve in students' vocabulary mastery. student involvement during the implementation of the crossword puzzles went well, the application of crossword puzzles and complementary actions succeeded in helping students understand new words and learn English better. In conclusion, students' vocabulary mastery was improved through the application of crossword puzzles.

Key words: Crossword Puzzle, Vocabulary mastery, Classroom Action Research