It is necessary to analyse normal Korean sentences for constructing a Korean sentence grammar, and in that regard the ellipsis phenomenon is a matter of the utmost importance. Ellipsis is defined as a phenomenon where a sentence element is omitted although it should be present in the sentence. A sentence in Korean where a subject is missing has no subject, not because it is omitted, but because it is not needed. Ellipsis is different from contraction which curtails the length of word or sentence. A sentence with ellipsis appears abnormal in terms of its syntax and semantics, which makes it possible to distinguish between elided sentence and normal sentence. Concrete examples of ellipsis concerns ellipsis of the modified in the modification construction and that of the verb ‘hada’, and several other types. It is believed that there are many more examples besides those examined in this paper. It is suggested in the paper that ellipsis is motivated not for the purpose of utterance economy, but for the purpose of communicative efficiency (or transparency). Due to the motivation and some other basic traits of Korean syntax, ellipsis takes place in the modification construction, etc. Ellipsis is a universal phenomenon, but the omittable object, the way of ellipsis, and its condition demonstrates language-particular characteristics. In particular, Korean and Indo-European languages show difference in sentence structuring, and thus in details of ellipsis.