The purpose of this study was specifically focused on the lower garment sizing system of adolescent apparel, especially on trousers for adolescent boys. Research Method was performed by using the physical measurements of adolescent boys (14～19 years old, n=1,344) based on the ‘he fifth Korean national physical standard reports’ surveyed by SIZEKOREA. The basic materials for this study are obtained from the observations of physical characteristics and classification of physical figures based on the characteristics from waist down. The results from this study are as follows. 1) according to the analyses of physical changes, most items including height, circumference, length, breadth and depth were increased at the age of 14～18 and gradually reduced at the age of 19. 2) The factors which compose the lower body resulted were appeared that the first factor was vertical factor, the second factor was horizontal factor, the third factor was hip length, and the last factor was buttlock-popliteal length. 3) Formalization of physical figures for adolescent boy's lower body analyzed from factors grouped three types. 4) The basic parts that we used to propose the apparel sizing system distinguished by physical figures were Waist Circumference (Omphalion) (every 3cm regular gap) and Hip Circumference (irregular gap). Physical figures assigned 5 to Type T, and 4 to type M. The reference measurement items were divided into 7 items which are correlated with apparel manufacture.