In spite of the proliferation of research studies on vocabulary knowledge, investigating the relationship between self-regulation, vocabulary size, and collocational knowledge among English as a Foreign Language (EFL) learners has received scant attention. The current study aimed to investigate whether vocabulary and collocation size can explain EFL learners' self-regulated vocabulary learning. A population of 271 EFL learners from three state universities located in Iran participated in taking lexical measures (VST, Lex30, and a collocation test) and filling a questionnaire (SRCvoc). To check the relationship between self-regulated vocabulary learning, vocabulary and collocation size, standard multiple regression was conducted with SPSS. The results of standard multiple regression analysis showed that EFL learners' vocabulary and collocation size could explain a significant portion of the variance in the score of their self-regulation in vocabulary learning. The findings also indicated that verb-noun collocation size and productive vocabulary size were significant predictors of EFL learners' self-regulated capacity in vocabulary learning. Finally, implications and suggestions for future research are offered.