The tourism industry of South Korea has grown steadily and especially Chinese tourists made a huge contribution showing a rapid growth rate of visits in recent years. Despite the increasing number of Chinese visitors, there are still many inconveniences for them and one of them is of public transportation use. For this reason, this research aims at studying why they feel uncomfortable and difficult to use the public transportation and how to fix the issues. More specifically, the UI design of subway ticket machine in Seoul Metro was studied because it is the most popular means of transportation for them. The literature review of mental model theories was preceded to study the relation of users and UI design which was followed by contextual inquiry as a qualitative evaluation methodology of finding out ‘why’ and ‘how to fix’ issues. It was based on the observation on the subjects’ actions and inquiry in the natural environment and then the data was analyzed by sequence model to specify breakdowns of mental model mismatch between the users and the UI design. In turn, the data was analyzed based on an affinity diagram. As a result, it was found from this research that the most common issue was unfamiliar design and process for the users. Also, the issues of feedback were found to play a crucial role as well making them to fail or face severe errors even though they were less frequent issues compared to the others. Therefore, the feedback with a consistent language and easily recognizable design, and the layout and information architecture of the interface according with the mental model of Chinese users could be deducted as the guidelines to improve UI design of subway ticket machine in Seoul Metro.