Knowledge of English affixes plays a significant role in increasing knowledge of words. However, few attempts have been made to create a valid and reliable measure of affix knowledge. The Word Part Levels Test (WPLT) was developed to measure three aspects of affix knowledge: form (recognition of written affix forms), meaning (knowledge of affix meanings), and use (knowledge of the syntactic properties of affixes). A total of 118 derivative affixes were selected based on frequency data from the British National Corpus. First, data was collected from 417 Japanese university students to revise poorly working items using Rasch analysis. Second, the responses of 1,348 people representing more than 30 different native languages were analysed to determine the affix difficulty levels. A description of the test, justification for its design, and practical implications are provided.