This study focuses on the medieval Mongol costume called YosunOja. This dress consists of a blouse and a skirt, and some lines on the waist part. The purpose of the study is to analyse the structural characteristics of this costume, especially on the medieval remains from the tomb Mingshui in the district of Neimenggu. It can be analysed that this costume has three structural characteristics, which are the functional structure, sumptuous one, and the idea of Buddhism. The functional structure consists of the style with a blouse and a skirt, which gives enough surplus for the movement, the back-slash for the ease of the riding, and the dart in the front line which eliminates unnecessary surplus. The sumptuous structure is the many lines of the waist which show the maximization of the decoration, the extraordinary long sleeves, and the use of the expensive gold brocade which shows the wealth of the dresser. And also, the cutting of the fabric into the numerous segment may mean the medieval Mongols believed in Buddhism.