This study examined the effects of self-efficacy on language learning strategies by focusing on vocabulary learning strategies (VLSs). A group of 281 EFL learners from two universities participated in the study. They completed the Vocabulary Size Test (Nation & Beglar, 2007), questionnaires on self-efficacy, and an open-ended question about their use of VLSs. The learners were divided into three groups based on their responses to the self-efficacy questionnaire. The effect of self-efficacy was then examined by utilizing text mining. The results show that the effects of self-efficacy were observed in the participants’ open-ended responses. It also became clear that those with high self-efficacy were active users of VLSs, they employed deep strategies, and they were metacognitively superior to participants with medium and low efficiency. Those with medium self-efficacy were also active users of VLSs, but they used shallow strategies compared with the high self-efficiency group. Those with low self-efficacy tended to be passive users of VLSs. The pedagogical implications of the current study are discussed mainly in terms of incorporating self-efficacy and self-regulation enhancing instructions into vocabulary teaching.