This research examines the effectiveness of using Kamusku application dictionary to the students’ vocabulary mastery. Kamusku application is an application dictionary that consists of bilingual vocabulary and some examples, pronunciation, some idioms and Indonesian translation. When the smartphone connects to the internet, the application can give more information about the usage of the words. This research is experimental research. The experimental class of this research is X TKJ 2 and the control class is X TKJ 1. Each class has 30 students. The researcher gave the application to the experimental class and asked students to use the application continuously. Before doing the research, the researcher tested the students’ vocabulary and knowledge in their English lesson. The researcher got the equal average of each class. Using average test, the researcher got t_count is -0.425and t_table is 2.285. After doing the treatment, the researcher found that the average of experimental class is 83.333 and the average of control class is 71.833. The equal quantity of each class is 30:30, using α= 5% with dk = 30+30-2 = 58 got t_table is 1.6716 and t_count is 5.271. The quality of t_count > t_table, and there is differentiation average value of the control class’ students and experimental class’ students. The experimental class has the higher average than the control class. Using N-gain test, 0.4 is in medium criteria. It means that the use of Kamusku application dictionary is effective enough to the students’ vocabulary mastery.