Vocabulary is one of the important aspects in the process of learning English. It is a basic component of the four main language skills that can improve one's mastery of English. There are many application media for English which can help students in learning activities. This study used the Duolingo application as the object research. The purpose of this research is to investigating the implementation of the Duolingo application to assist EFL students of State Islamic University Prof. K.H. Saifuddin Zuhri Purwokerto in learning vocabulary. In this study, researchers used qualitative data analysis techniques to obtain actual data. The method used to analyze the data is thematic analysis in order to gain experiences, views and opinions of participants in using the Duolingo application. For collecting the data, researchers used observation and interview. The finding of this study were about the perceptions and students interest towards Duolingo application as learning media which assist them in learning vocabulary. It revealed that most of participants gave positive statements about the Duolingo application they used. they think this application helps them in improving vocabulary especially when they as beginners. Some of them also gave statements that Duolingo would be a tedious application to use when the learner have a great vocabulary level.