This paper investigates the L1 acquisition of unaccusativity by 5-year-old and 6-year-old Korean children, using an oral grammaticality judgment task. The children``s knowledge of the unaccusative-unergative distinction was tested with respect to the resultative -a/e iss- and the degree adverb keuy, both of which are available exclusively with unaccusatives. The results show that the children recognized that the resultative and keuy were grammatical with unaccusatives, but not with unergatives. This suggests the distinction is present in child grammar around the age of 5. This paper also examines the role of the morpheme ci-, argued to be a morphosyntactic reflex of unaccusativity in Korean.