The aim of this study is to investigate the relationship between morphological awareness and vocabulary knowledge of Iranian high school students. Nation's Vocabulary Level Test (VLT) was used to test students' knowledge of words drawn from the 2000, 3000 and 5000 most frequent occurring word families. Two morphological awareness tasks (a morpheme identification task and a morphological structure test) were also used to assess students' morphological awareness. The VLT results reveal that the students performed better at the 2000 level than the two higher frequency levels. There existed a significant relationship between the learners' performance on the vocabulary level test and the morphological awareness tasks. These findings implicate the importance of facilitating the students' morphological awareness in English vocabulary learning for EFL learners in Iran.