This research focuses on the use of smart phone applications in learning English vocabulary of 400 first-year elementary students at Vietnam National University, Hanoi – International School (VNU-IS). It aims to find out: which smart phone applications are most frequently used to learn English vocabulary by these students, how effective the use of these applications is in improving students' vocabulary basing on the results of the pre-test and post-test taken before and after the course, how the students evaluate these applications in terms of their design, functions, ease of use, variety of vocabulary tasks provided, the interest and motivation that they bring about to students, the thing(s) that students dislike most about these applications and the students' recommendations towards the use of these applications. From the data collected and the findings, some suggestions are made in order to improve the effectiveness of smart phone applications in improving English vocabulary of first-year students at VNU-IS.