Learning English need vocabulary mastery to understand. Vocabulary can be influenced by age, gender, ability, family, social status, as well as psychological and geographical factors. This causes differences in a person's ability to master and understand vocabulary. The way to release the shackles of limited vocabulary mastery is to be aware of these limitations and hone your abilities independently. Therefore, researchers are interested in conducting research as well as disseminating independent learning methods to master vocabulary. Because nowadays everyone uses smartphones like school children, researchers use them to conduct research. This research will describe the perceptions of students and teachers regarding the use of the Duolingo application for learning English especially for vocabulary mastery at Nurul Islam Junior High School Semarang. In collecting and analyzing data, this research uses qualitative methods with descriptive qualitative data research design. Researchers uses purposive sampling. Based on data analysis, the research respondents were 25 grade 8th students at Nurul Islam Junior High School. Data collection uses interviews and questionnaires. After monitoring students using the Duolingo application to learn English for a week, it was found that there was an increase in motivation, addition of vocabulary, and also confidence in learning English. It is hoped that this research can provide the following meaning so that English teachers can help students master English. Researchers will gain experience and knowledge of what will be done. For students majoring in language or literature, using the Duolingo application can help students in learning English.