This study investigates lexical development of Thai children at 0;9-2;0. Based on the MacArthur Communicative Development Inventories: Words and Gestures, 1989 (CDI), a Thai version of the CDI word checklist was developed. Following a cross-sectional research design, parents of 180 children were asked to report their children’s lexical development. Four main aspects of lexical development were investigated: the onset of development, cumulative vocabularies, lexical categories of words acquired and the relationship between comprehension and production in lexical acquisition. 

It was found that individual variation plays an important role in children’s lexical development. Instead of pinpointing the exact age of the onset of development, a tentative range of onset is reported about 0;9-1;3. Although children’s vocabulary items increase over time, it was found that there were two distinct types of increase: gradual and rapid. The gradual increase was found from 0;9-1;6, and 1;9-2;0. The rapid increase, at about 1;6, represents a period of vocabulary explosion. It was found that Thai children’s lexicon is “noun” dominant. From an examination of the relationship between word comprehension and production, it was found that comprehension precedes production at the onset of lexical development. However, data from this study also demonstrate that at about the period of vocabulary explosion, some words are comprehended only after they have been produced. It is also suggested that lexical development should be investigated using a longitudinal research design, because data from a longitudinal study can indicate the development of individual children and might demonstrate more clearly development in terms of individual variation.