Osgood-Schlatter Disease (OSD) is an apophysitis caused by rupture of the growth plate at the tibial tuberosity. The condition occurs in active boys aged 12-13, coinciding with periods of growth spurts. However, the risk factors of OSD remain unclear. The purpose of this study is to investigate the relationship between the lower limb length and the onset of OSD. The subjects of the present experiment were 27 male junior high school soccer players whose femur and tibia length were measured by using the scanned image of DXA (Delphi-AQDR ; Hologic) twice. In addition, we performed a physical examination about the medical history of Osgood-Schlatter Disease. The subjects were divided into two groups ; the sign and symptom of OSD started in the studying period (OG ; 39 legs) or not (NOG ; 12 legs). As a result of logistic regression analyses, the increase of femur length proved to be a causative factor of OSD. In addition, the increase of femur length was significantly larger in OG than in NOG. Larger increase of femur length would elongate and tighten the quadriceps muscles, resulting higher tension and rupture of growth plate at tibial tuberosity. It has been known that OSD occurs at apophysial stage, when the cartilage tissue of growth plate is mechanically fragile to higher tension of the quadriceps muscles. These results suggest that larger increase of femur length can cause the onset of OSD.