The study was about The use of Graphic Organizer to improve student’s vocabulary mastery. The objective of this study is to find out the effectiveness of Graphic organizers to improve students' vocabulary mastery. Quasi experimental design was used in this study. In addition, quantitative data research was used in this study to prove the result of this study. The population of this study was eleventh graders of SMA Sultan Agung 3 Semarang in the Academic Year of 2017/2018 with total students was 130. The sample were XI IIS 1, with 36 students chosen as experimental class, and another class is XI IIS 2, with 35 students chosen as control class. Those sample were taken by using convenience sampling technique. Researcher used this sampling technique because the English teacher order to use XI IIS 1 and XI IIS 2 as sample. Test in a form of multiple choices was used in this study for collecting data from the sample. The result of this study showed that the means score of experimental was 19.91 and the means score of control group was 20.69. After the treatment was given for four meetings, the mean score of experimental class increased to 21.92 and the means score of control group increased to 2.116. The result of the Independent Sample T-test of post-test showed that t-test for Equality of means sig. (2-tailed) was 0.000 (less than 0.05). It means that H0 was rejected and H1 was accepted. So, it could be concluded that the Graphic Organizer was effective to improve student’s vocabulary mastery.

Keyword : Graphic Organizer, Vocabulary, Vocabulary mastery.