The purpose of this paper examines the following: (i) certain differences exist regarding degrees of grammaticalization between Korean and Japanese, and (ii) what factors cause the differences. The arguments of this paper are as followed: a. The degrees of grammaticalization of Korean are lower than the degrees of Japanese concerning 6 grammatical categories: negative sensitive items, aspectual markers, sentence-final forms, compound verbs, compound postpositions, and auxiliary verbs. b. The phenomena could be caused by the following three factors. The first factor is morphological and syntactic differences between Korean and Japanese. The second one is the discrepancy of the linguistic history as it appeared in the early Korean and Japanese literatures. The third factor is differences of language performance and socio-cultural cognition.