This study explores the elementary school English underachievers` experiences of using SMART devices in their vocabulary learning. Five students enrolled at a local elementary school in Busan participated in the study. As a main tool of inquiry for the study, a qualitative case study was conducted using classroom observation and in-depth interviews as well as vocabulary tests. The qualitative data were coded and analyzed by NVivo version 10.0. The data suggest that the participants enjoyed the vocabulary learning using SMART devices and felt interested and motivated in vocabulary learning. They also appeared to be engaged in self-directed learning more actively and displayed a positive attitude. Furthermore, the self-directed vocabulary learning experiences were provided through enough practice opportunities, audio-visual language input and prompt feedback. Each participant gained a different level of achievement in his receptive and productive vocabulary abilities. Based on the findings of the study, some educational implications can be made that underachievers should be taught by an individually tailored program using effective learning tools. In-service teachers also need to consider SMART devices as teaching tools. More research based on the classroom settings should be conducted and suggests effective ways to use SMART devices in the language learning.