Many researchers think that the knowledge of using affixes for vocabulary learning is one of the most effective strategies (Nation, 2001). This study presents an attempt to find out the relationship between the affix knowledge and vocabulary building. And how it is vitally important as a systematic means of increasing vocabulary. The problem is that the topic of prefixation and suffixation is not taught in a systematic way to secondary school learners. Therefore, students at this level face difficulties in identifying derived words when they come across them. Data for this study was collected by using two tools, a written test for secondary school learners and a questionnaire for language teachers. The results show that the affix knowledge does play an important role in language learning, and it has a positive impact on vocabulary building. It also shows that teachers generally have a limited focus on the teaching of the prefixes and suffixes however, by incorporating systematic teaching into a lesson, we can enforce the students’ vocabulary. Teaching affixes systematically will help the learners to a certain amount whether they have knowledge of base forms of vocabulary items or not. Moreover, the results show that female students have more knowledge of affixes than their male counterparts.