This research examined the function within lexical access of the main morphemic units from which most Arabic words are assembled, namely roots and word patterns. The present study focused on the derivation of nouns, in particular, whether the lexical representation of Arabic words reflects their morphological structure and whether recognition of a word involves recognition of its morphological constituents. The morphology of Arabic language constitutes two main systems: inflectional morphology and derivational morphology. Inflectional morphology is used to express grammatical relations and derivational morphology deals with the formation of new words in language; it contains two types of word patterns: verbal and nominal. In this study, it was assumed that roots are lexical entities that can facilitate lexical access to a large cluster of words that derive from them, whereas word patterns are not lexical entities and have no role in access to words assembled by them. The lexical representations of the main morphemic units were examined in four experiments using skilled readers applying masked priming technique tasks: lexical decision and naming. The results indicated that previous exposure to a word sharing the same word pattern had no marked facilitating effect on lexical decision or on naming. The main conclusion of this study on the morphology of Arabic is that roots and word patterns have no essential role in word organisation in the mental lexicon. In addition, words in nominal pattern in derivational morphology are represented in their whole shape in the mental lexicon and each word has an independent representation.