Background: Osteosarcoma is the most common primary bone tumor in childhood and adolescence. Carboplatin, a platinum-derived agent, is used as neoadjuvant chemotherapy for pediatric osteosarcoma because of its anti-tumor activity and had low toxicity as compared to cisplatin. Objective: To determine demographic data, prognostic factors and outcome of childhood osteosarcoma treated with a carboplatin-based chemotherapeutic protocol at Chiang Mai University. Method: A retrospective analysis was conducted on 34 osteosarcoma patients aged less than 18 years and treated between 2003 and 2011. Results: Overall limb-salvage and amputation rates were 23.5% and 70.6%, respectively. With the mean follow-up time of 29.5 months (1.5-108.9), the Kaplan-Meier analysis for 3-year disease-free survival (DFS) and 3-year overall survival (OS) were $20.2{pm}7.7%$ and $47.1{pm}9.5%$ respectively. Patients who had initial pulmonary metastasis were at significantly greater risk for developing recurrence (p=0.02, OR=7; 1.2-40.1) and had a tendency to have lower 3-year OS compared to those without initial pulmonary metastasis ($28.1{pm}13%$, $63.1{pm}12.3%$, respectively, p=0.202). On univariate analysis, age at diagnosis >14 years and patients who were declined surgery were significantly associated with lower 3-year OS (p=0.008 and <0.05, respectively). However, age at diagnosis, sex, tumor size and histological subtypes were not found to significantly affect recurrence or survival. Conclusions: In our study, the survival rate was far lower than those reported from developed countries. These might indicate the ineffectiveness of carboplatin in combination with doxorubicin as frontline treatment of pediatric osteosarcoma, especially in those with initial pulmonary metastasis. Refinement in risk and treatment stratification and dose intensification for pediatric osteosarcoma constitutes a future challenge to improve outcomes, especially in metastatic patients who may need a more intensive regimen.