Self-regulation is referred to as learners' self-generated ideas and actions which are systematically directed towards achieving educational goals and require learners' active participation in the learning process (Zimmerman & Bandura, 1994). The present study investigated the relationship between Iranian EFL students' self-regulation capacity for vocabulary learning and their vocabulary size. For this purpose, the researchers made use of two main instruments: the self-regulation capacity in vocabulary learning scale developed by Tseng et al. (2006) consisting of five subscales of commitment, metacognitive, emotion, satiation and environment control, and a bilingual vocabulary size test developed and validated by Karami (2012). The results of the data analysis revealed no significant relationship between the two variables measured by these instruments. However, the results of the multiple regressions indicated that the metacognitive control compared to the other subscales made a better contribution to the prediction of learners' vocabulary size. In addition, based on the analysis of variance (ANOVA), which examined and compared the self-regulatory strategy use of learners in different experience groups, the first year students had a higher mean score in their self-regulation capacity, which can possibly be attributed to the strategies they have learnt in their Study Skills courses. Finally, it was suggested that teachers must try to develop self-regulatory power in the learners because their creative effort and informed decisions in trying to improve their own learning are highly important.