In order to clarify the reason why breaking occurs at the special breaking position (BP) within the broken internode, morphological characteristics of BP were compared with those of other areas within the same internode. In the broken internode of lodging-susceptible cultivars, hardness of the internode (IN) and dry matter accumulation was lowest at BP. It seems that the two apparent tendencies are partly due to the increase in the flattening of internode of the area by tight wrapping of the leaf sheath, the decrease in both the thickness and the supplementary strengthening efficiency of the leaf sheath in this area. All these aspects together indicate that BP is the weakest area of the internode which results from the shortcomings of both the internode and the leaf sheath in the area. These results illustrate the reason why BP has the lowest breaking strength in this area which leads to breakage.