The aim of this study is to investigate the characteristics of pre-existing physical factors associated with the onset of Osgood-Schlatter disease (OSD) in junior soccer players. Fifty-nine junior soccer players of the elementary school were included in this study. The boys who had been diagnosed as OSD were excluded. At the time of the first investigation, the subjects were examined through physical evaluation and an ultrasonography. The subjects were followed up one and a half years, and the tibial tuberosity was classified according to the growth stages using the ultrasonography. After the end of follow-up period, the subjects were divided into two groups, those with signs and symptoms of OSD that appeared during the follow up period and those without that. The Mann-Whitney U test was used for the comparison between OSD group’s and control group’s initial assessments. The players who were followed up for one and half years were 38 people. 5 knees of the 4 players were clinically diagnosed as OSD (OSD group), and the remaining 34 players with no symptoms of the OSD constituted the control group. The range of the bilateral hip external rotation, straight leg raise of the non-dominant side, and knee flexion in the OSD group were significantly lower than those in the control group (p < 0.05). We concluded that a decrease in specific joint flexibility might be related with the onset of OSD.