Neoplastic neovascularization is regulated not only by stimulators, but also by inhibitors of angiogenesis and might be the result of a net balance between the positive and negative regulators. Endostatin (ES) is a potent inhibitor of angiogenesis. The expression of ES has not been investigated in patients with osteosarcomas (OSAs). The aim of this study was to determine whether there is a correlation between the expression of ES and clinicopathologic parameters and/or outcomes in patients with OSAs. We made tissue microarrays from 46 cases of OSA and analyzed the expression of ES using immunohistochemistry. Staining was assessed in a semi-quantitative manner by scoring the proportion of positive tumor cells over the total number of tumor cells. A sample was defined as ES-positive when 10% or more of the tumor cells were stained positively throughout the tumor core. ES was localized to the cytoplasm of the tumor cells. 32.6% (15/46) of the patients were ES-positive. The expression of ES was positively correlated with tumor size (p = 0.011), histologic grade (p = 0.034), stage (p = 0.025), and distant metastasis (p = 0.036). Our results suggest that the expression of ES is increased in OSA, and ES may be used as a prognostic marker in patients with OSAs.