The Korean expression kes has various grammatical functions, being used in various contexts. One of its main uses is to introduce a complement clause. This paper examines grammatical functions of the kes complementation clause based on a corpus study. The paper also looks into the similarities and differences between the kes complementation and the -um or -ki nominalized construction. Our research indicates that the syntactic constraints of the kes complementation clause is closely related to the semantics of the predicate in a higher clause as well as its factivity.