The aim of this research is to know whether there is a correlation between motivation and vocabulary size. The method used in this research was correlation method. This research was carried out at UniversitasMuhamadiyahPurwokerto in academic year 2017/2018. This research used random sampling technique to take 30 students as the sample. The writer collected the data using questionnaire and test. The questionnaire was used to collect the data about students’ motivation. The test was used to collect the data about students’ vocabulary size. The data was analyzed using Pearson’s product moment correlation formula. The result of the correlation coefficient calculated was 0.082 at the level of significance 5%. This indicates that there is no significant correlation between students’ motivation and vocabulary size, because the rtable coefficient is 0.361, which is higher than the calculated coefficient. In conclusion, there was no correlation between motivation and vocabulary size of the sixth semester students of Muhammadiyah University of Purwokerto.