The researcher has worked as a Japanese cram school teacher in Taiwan for nine years since 2002. In these nine years, the researcher has encountered different kinds of students and earned myriad of experience in this path. The researcher found that Japanese learners in Taiwan, no matter what age or what level, made almost the same mistakes. The commonality of making errors caused the researcher¡¦s curiosity. The researcher took this opportunity to seek for the reasons that caused these common mistakes. One of the most common mistakes they made is the use of the ¡§particle¡¨. Japanese¡¦s particle was classified neatly, especially ¡§prepositions¡¨ which were the most difficult to learn. When students first learn prepositions, they always get confused; the catachreses of the "De (で)" and "Ni (に)¡¨ were often brought to discussion. In the previous studies, many scholars have focused on this topic and done many researches (Sakota, 1998;Sugiyama, 2002). Based on those studies, with the writer¡¦s practical experience, she analyzed and probed the misuse of De and Ni through testing students. In the test, the researcher differentiated the students into three levels to see how different levels of students react to the use of prepositions, De and Ni ; the researcher then analyzed if there were any special tendency to the use of those preposition from the collected data. The testing method was to ask each levels of students to fill in the blank of a preposition quiz and then interview them about why did they pick those certain preposition to fill in the blank. Using fill-in the blank tests, the researcher found that the student's frequency of use of De and Ni and the frequency of incorrect answers were different. Through introspective investigation, the researcher found that the student's thinking process when using De and Ni are "Noun+ preposition" , "verb+ preposition". After interviewing the students, the researcher found that most subjects have a stereotyped "image of place or name of building + De", and "position or abstract places + Ni". The researcher hoped to understand the reason of the preposition mistake, De and Ni, in Japanese leaner of Taiwan by the research and find the solution of this common mistake. The writer wished that this could benefit not only teachers but students and provide them directions for teaching and learning.