Abstract Introduction: In the related literature, knowledge of vocabulary is mentioned to be crucial as one of the crucial parts of language learning. Measuring learners’ vocabulary knowledge is regarded to be essential in that it provides both teachers and learners knowledge of the problematic areas and suggests some practical ways to improve the vocabulary learning process. It could be said that an autonomous learner is a leading actor in his own language learning process because, as stated by Nation (1998), learning is performed by the individual learner. Little (1995) asserts that learner autonomy should be set as an explicit goal in language learning contexts in that autonomy on the part of the learners plays a vital role in student success. So, we hypothesize that vocabulary learning autonomy has a significant influence on the learners’ vocabulary size. Methods: This study aims to investigate Turkish ELT student teachers’ vocabulary learning autonomy, vocabulary size, and the potential relationships between these two variables. Ninety-five student teachers in an ELT teacher education program at a university in Turkey participated in the study. The data were collected via two quantitative data collection instruments: Vocabulary Learning Autonomy (VLA) questionnaire and The Vocabulary Size Test (VST). The data were analyzed using descriptive statistics and correlations. As for the VST, the correct answers were counted, and an overall score was found. The participants’ scores were multiplied by 100 to get their total vocabulary size up to the 14th 1000 word-family level. As for the questionnaire, descriptive statistics (mean, frequency, standard deviation) were conducted. Results: The findings revealed that student teachers held a moderate level of vocabulary learning autonomy. The vocabulary size mean score was 77.14, which means that the participants had approximately enough vocabulary to deal with unsimplified written texts, and enough vocabulary to deal with unsimplified spoken texts. Additionally, there were significant and positive relationships between the responsibility and ability dimensions of the VLA scale. However, the findings revealed non-significant correlations among all the VLA’s dimensions and the vocabulary size. Discussion: The findings regarding the participants’ vocabulary learning autonomy showed that the participants held a moderate level of vocabulary learning autonomy. This finding is essential in that one of the most important goals of education is encouraging learners to work more independently both in and out of the classroom (Moir & Nation, 2002). There were significant and positive relationships between the responsibility and ability dimensions of the VLA scale. This finding, which is in line with Koller (2015), suggests that the participants feel capable of learning vocabulary items when they perceive themselves as responsible people instead of a teacher. Based on the results regarding vocabulary size, it is reasonable to conclude that the participants had approximately enough vocabulary for comprehension of unsimplified written texts and enough vocabulary for spoken texts based on Nation’s (2006) research reporting that learners need 8000 to 9000 word-family vocabulary for comprehension of unsimplified written texts and 6000 to 7000 word-family vocabulary for unsimplified spoken texts. Limitations: The research is limited to only first-year student teachers of English and the data consist of only quantitative data. Conclusion: The findings of the present study imply the need to foster vocabulary learning autonomy of learners in teacher education programs. Student teachers hold two identities as learners and teachers of the future. It is crucial to examine their readiness because there is a bulk of evidence in the literature that teachers’ readiness for autonomy affects their ability to foster their students’ autonomy. The findings of the present study imply the need to foster vocabulary learning autonomy and utilizing some strategies to improve vocabulary size.