Background: Unlike two known peak age groups (10 -14 and older than 65 years of age), there was relatively scant attention in osteosarcoma patients between them, and previous reports had analyzed other than Asians. Therefore we conducted this study with Korean patients and focused on 20 -40 years of age, then investigated age-specific or race-specific meaningful outcomes. Materials and Methods: 379 patients who were newly diagnosed as osteosarcoma in two Korean major cancer hospitals from September 1986 to July 2015 with more than 2-year follow-up were retrospectively studied. There were 290 patients with 20 -40 years of age, 60 patients aged 19 or younger, and 29 patients aged 41 or older. Age at diagnosis, gender, location of primary lesion, metastasis at diagnosis, histologic subtypes, histologic response were analyzed as prognostic factors by overall survival (OS) and event-free survival (EFS) rates. Results: Primary pelvic lesion and poor histologic response were significantly unfavorable prognostic factors, but not age-specific. However, proportion of metastasis at diagnosis was significantly highest in 20 -40 years of age group. For the age at diagnosis, older patients presented poorer 5-year OS and EFS rates, but for 10-year, 20 -40 years of age group showed unusually better prognosis than younger age's, with statistic significance in OS rates but not in EFS rates. Conclusion: Osteosarcoma patients with 20 -40 years of age in Korea presented relatively better prognosis after longer-term 10-year despite their late diagnosis causing more metastasis at diagnosis, which emphasized expedite diagnosis and appropriate treatment to expect much better prognosis than peak age groups.