There are many types of modifying function of Japanese Onomatopeia and a pile of related researches. One of the functions is to be used as a noun but the research about the nominalization of Onomatopeia is rare. Therefore, this paper is aimed to make a deeper study on the nominalization of Onomatopeia such as what kind of onomatopoeic terms are used as noun and how they represent, etc. And it comes to the conclusions as following: 1) when it functions as a noun, the appearance of kaku-joshi (case marker) will become an indicator of whether it is nominalized. 2) when it appears as an independent phrase, it refers to the subject of that phrase. 3) the onomatopoeia that represents the status of substance tends to become nominalized. 4) some onomatopoeic terms often appear as a noun and the pattern is fixed. 5) from the characteristic of phonology, the onomatopoeic terms with dissyllabic root are easier than monosyllabic to become nominalized.