This thesis is entitled The Effect of Using Tree Diagram Technique for Improving Students’ Vocabulary Mastery (An Experimental Research on Seventh Grade Students of MTs Miftahul Huda Bulungan in the Academic Year 2021/2022). The objective of this research is To find out the effectiveness of using Tree Diagram Technique to improve vocabulary mastery on seventh grade students of MTs Miftahul Huda Bulungan in the academic year 2021/2022. The researcher used Quantitative research with nonequivalent control group design. The researcher used non random sampling technique and involved two classes which is experimental class consist of 33 students and control class consist of 32 students. Based on the analysis that has been done, the researcher found there are differences of the students’ score between experimental class who get the treatment of using tree diagram trechnique and control class that does not get the treatment. Based on the data, the mean score in experimental class for the pretest is 61,69 and for the posttest is 83,09 while the mean score in control class for the pretest is 59,03 and for the posttest is 74,09. In addition, the result of T test of this research is 2,788>t-table 1,670. The conclusion of this research based on the data analysis that have been done, the researcher found that there are significant’ score of the pretest and posttest and the implementation of tree diagram technique is effective to improve the students’ vocabulary mastery.