The utilization of mobile technology among students has given birth to a new form of learning known as mobile learning (mLearning), where learning is supported by mobile devices, ubiquitous communications and intelligent user interfaces. There has been significant growth in mLearning research in the last half decade. The action research undertaken in this study was conducted to investigate how short message (SMS), which is a popular mobile application among tertiary students, is able to assist English as a Second Language (ESL) students’ vocabulary learning. The study set a dual goal: first, provide an mLearning service, in order to investigate the effects of a novel learning method, and students’ acceptance and usage of it. Second, modify and refine the mLearning strategy, in order to give students an opportunity to integrate mobile technology into their study seamlessly, to change or extend where and when they learn, and to “flexibilize” their learning interactions / conversations with teachers (or assistant teachers). The two-cycle study, conducted in an English for Academic Study (EAS) class at Auckland University of Technology (AUT), allowed the researcher to gather substantial information in order to achieve the research goals. Nine data collection techniques, including a reflective journal, interview, questionnaire, observation, informal conversation, students' word test records, SMS transmission history, a professional presentation and a portfolio, were used to gather data. Both deductive and inductive methods were employed to analyse data. The findings of the two research cycles demonstrated that SMS-based vocabulary learning was able to enhance ESL students’ learning by adding flexibility and providing a stronger motivation for study. The results suggest that mLearning contributed to an improvement in students’ learning. Student-centred design played an important role in the success of the integration of mobile technology into learning activities. The pedagogical content for mLearning can be greatly enhanced in the future.