There are several flaws in the research design of Receptive and Productive Vocabulary Sizes of L2 Learners by Webb (2008). The key flaw is related to the researcher’s understanding about the research topic, namely receptive and productive vocabulary. Webb first of all didn’t give a clear explanations or definitions about what receptive and productive vocabularies are. He did give a brief introduction about receptive and productive vocabularies by saying that receptive vocabulary will enable students comprehend a text or a listening task, while productive vocabulary will help students to speak and write. ---- This explanation is just a common sense to everybody, however, before making a research instrument, there should be more accurate and detailed explanations about what receptive and productive vocabularies mean and what the differences between them are. The explanation he initially gave is only about the function of receptive and productive vocabularies. This deformed explanation of the key terms associated with the research demonstrates his confusion over receptive and productive vocabularies, which is proved by his later critiques on the instruments employed in the previous studies.