To develop grammatically suitable English, learners need to have the ability to use the appropriate forms of a word in the context. If they do not have adequate knowledge of the four major word classes, they are either forced to use only the forms they already know or use a totally different word that is appropriate to the given grammatical context. It may be assumed that when a learner knows one member of a word family (e.g., produce), the other members (e.g., production, productive) are relatively easy to learn. Although knowing one member of a word family facilitates receptive mastery of the other members, previous studies indicate that English learners often face significant problem producing the four different derivative forms (i.e; noun, verb, adjective, and adverb) within a word family. To explore the issue, the present study examines the productive derivational knowledge across members of a word family for 16 prompt words and in relation to more global knowledge of the target words. A total of 93 postgraduate and undergraduate English language teaching students at Shiraz AzadUniversity, and Marvdasht Azad University participated in the study. A self-rating four-stage developmental scale and a derivative elicitation test were used to direct the research questions. The results of the study showed that nouns and verbs were the best known family members with adjective and adverbs being less known. The subjects showed derivative knowledge of two of the four possible derivative forms for each word family. In addition, as the global knowledge of word knowledge increased, the productive derivational knowledge also increased steadily. Some suggestions are offered to facilitate the teaching of derivative forms.