Abstract: The present study investigates the relationship between derivational morphemes and breadth of vocabulary knowledge. The participants consist of 130 university students. So as to determine this relationship a two- section test is used: the X-Lex Vocabulary Size Test and Derivational Morphemes Test. The main findings of this study empirically indicate that the overall size of the student’s vocabulary knowledge is approximately 4014 word-families. As for the second section of the test, the findings showed that the majority of students revealed a definite knowledge in using these 22 derivations. The correlation tests proposed that there is a positive significant relationship between the two variables. Additionally, the result indicated a positive impact of gender on both components of vocabulary knowledge. This research has been concluded by suggesting some pedagogical solutions on different parts and stressed further research guidelines.Abstract: The present study investigates the relationship between derivational morphemes and breadth of vocabulary knowledge. The participants consist of 130 university students. So as to determine this relationship a two- section test is used: the X-Lex Vocabulary Size Test and Derivational Morphemes Test. The main findings of this study empirically indicat...Show More