This study’s objective is to find out if the Duolingo Application could aids students to improve their vocabulary mastery especially adjective. This research is quantitative and using pre-test and post-test design through a written test. The subject of this study were 31 students from SMP Negeri 1 Manganitu class 8 B. According to the data obtained, the mean Pre-Test score is 43,32, with the greatest Pre-Test score is 83 and the lowest is 10, and the highest Post-Test score is 97 and the lowest is 63, and the Post-Test’s average score is 79.80. Significant improvements can be noted in this data, and it can be concluded that the use of duolingo application successful for increase students’ vocabulary mastery