Many linguistic studies reveal that sign language is a fully-grammaticized language, not a pantomimic communication system. This paper reviews the results of investigation for sign language acquisition and examines how sign language studies contribute towards explaining the human capacity for language and the limits of language learning. First, the human capacity of language creation was discussed by reviewing the researches of deaf people with no or inconsistent language input. Second, this paper discussed whether the late language acquisition affects the ability to produce and comprehend a number of syntactic and morphological structures, by assessing sign language abilities of Deaf people who has various language environments. The results showed that the outcome of the sign language was sensitive not about the timing to expose to sign language learning but about the timing of the first language acquisition. Finally, sign language acquisition process in Deaf children was compared to spoken language acquisition process in hearing children. Language modality plays a very minor role in how children acquire language because the developmental course of language is very similar in two modalities. The above teaches us about the nature of human language and cognition.
