Early vocabulary ability is an important indicator of a child's linguistic, reading and cognitive development in later stages. Therefore, exploring the important factors which influence early vocabulary learning process could provide critical information to vocabulary instruction or intervention programs for children with normal language development or specific language impairment. This paper first addresses vocabulary development in children with or without language impairment, and then discusses the relationship between children's different levels of phonological processing abilities and their vocabulary development based on the framework of word learning process.