This study analyzed the factors causing the angular deformity as the tibia fracture below isthmus were treated with interlocking nails. For this purpose, we observed 70 cases of tibia fractures in the Gil General Hospital as subjects. Of the 70 cases under the treatment, 32(45.7%) showed the angulation of over one degree, and 11(15.7%) showed that of 5-10 degrees in any plane. According to the outcomes of this studies, the degree of angulation seemed to have close relation to the length of fracture line. Also, if the fracture line reached to the isthmus, angulation was apt to be caused. Even though most angulated cases had concomitant fibular fracture, fibular fracture, itself, seemed to have no direct relation to angulation. In consequence, the intramedullary interlocking nailing needs special carefulness in the case of breakage of cortical buttress such as long fracture line, butterfly fragment and fracture including isthmus.