Vocabulary is one of the English language components that students should be master in order to master the four English skills. However, students in high vocational schools have difficulty in remembering and understanding the meaning of the words due to the teaching methods and materials are not attracted. Online quiz application is one of the technology which matches with students’ ability in this digital era. Therefore, this study aims to investigate the effectiveness of Kahoot! application in improving diploma 1 students’ vocabulary mastery and to investigate the factors that influence students’ intention to use Kahoot! application. The data were collected from diploma 1 students in high vocational school in Bangkok who study English for communication subject through pretest, posttest, and questionnaire. The participants were asked to do pretest before Kahoot! as a treatment was used in the class and posttest was given after Kahoot! has been implemented in the class. Questionnaire was given right after the posttest. The findings show that using Kahoot! application has significant effect in improving students’ vocabulary mastery and competitiveness (CO), enjoyment (EN), and challenge (CH) as gamification elements have significant effect on students’ opinion of Kahoot! application.