This paper examined the grammatical category of modality in Korean. It is not an easy work for adult foreign learners of Korean to learn every single modal endings in their utterance. Even it is not clear-cut for Korean teachers to teach how to use Korean modal endings. Native speakers of Korean do not have to learn it since they can intuitively acquire the notion of the modality. In light of this, the main purpose of this study is to analyse the modality in Korean. The modality is defined as a grammatical form of the quality or state in question. These include the assertion or denial of any degree or manner of affect, belief, certainty, desire, obligation, possibility, or probability on the part of the utterer. The notion of the modality was classified as epistemic modality and deontic modality. The epistemic modality was effectively described through the literature review for teaching Korean as a Foreign Language. The deontic modality, however, was not sufficiently accounted for. Therefore, what is also discussed is the distinction of ‘-eulrae’ and ‘-eulge’ to better understand for the deontic modality. The empirical study was performed through the corpus of KAIST. From the data, a total of 602 tokens was acquired: 301 tokens for ‘-eulrae’ and ‘-eulge’, respectively. The most important findings of this study were that ‘-eulrae’ has the functional meaning of [suggestions](50.5%), [threat](19%), and [desire](13%), Otherwise, the results revealed that ‘-eulge’ has the meaning of [promise] (80%) and [one’s will for here and now](20%). (Hankuk University of Foreign Studies)