Vocabulary is regarded as an important part of communicative language ability and a significant predictor of second language proficiency. The relation between vocabulary knowledge and communicative language ability has important implications for L2 students, foreign language teachers and educational systems. Thus, many studies have been conducted to investigate this relation. However, there is scarce literature related to different factors which affect EFL learners' vocabulary knowledge. Hence, this study explored the relationship between motivation and vocabulary knowledge. 711 preparatory school students participated in the study. Language Learning Orientation Scale and Vocabulary Levels Test (VLT) were utilized in the data collection process. Participants emphasized the pragmatic use of English (extrinsic motives) rather than intrinsic motives, so findings revealed a moderate level of self-determination. The results of the vocabulary levels test showed that the mean score of the total vocabulary knowledge was below the average. On the other hand, a significant relationship was found between learners' motivation and their vocabulary knowledge. The results implied that language teachers should pay attention to their learners' motivation levels in order to increase their vocabulary knowledge.