Background and Aim: Premature birth and low birth weight are regarded as two risk factors for impaired language development. Hence, information about primary period of language development in these children is important for early detection of childrens needs during their language development period. The goal of this study was to determine the expressive lexicon size in 18 to 36 month-old premature children with low birth weight and to compare them with their full-term peers.Methods: In this prospective historical survey, using form II of the MacArthur-Bates Communicative Development Inventories (CDI), expressive lexicon size of 42 premature children with low birth weight who were 18 to 36 month- old and 42 full-term peers in three age groups with six month-intervals is evaluated. Forms were completed individually by mothers of children.Results: Data analysis showed that the difference in expressive lexicon between the two groups was significant (p=0.025). Nonetheless, in each group the difference in expressive lexicon size between the three age groups was not significant.Conclusion: The results of this study suggest that expressive lexicon size in premature children with low birth weight was smaller than their full-term matches and the development of lexicon size is delayed in these children. However, by increasing age, these children follow a similar development pattern in terms of lexicon size and eventually catch up with their full term counterparts. This study shows the importance of early intervention to expedite this compensatory mechanism in these children.