This study aimed at measuring the size of vocabulary knowledge of English as a Foreign Language (EFL) among undergraduate Jordanian students at different Jordanian universities. For this purpose, research questions were presented and the answers to these questions were provided and discussed. The participants of this study consisted of 39 undergraduate Jordanian students. A vocabulary test was used by the researcher to investigate the vocabulary storage of undergraduate Jordanian students. The results of the study indicated that the vocabulary size of Jordanian undergraduate students is fairly high. This is particularly true with the first few thousand-word list: 2000 and 3000-word list. However, for the 5000-word list, 10000-word list, university students have a lack of vocabulary storage which normally resulted in many difficulties in English language skills.