B＞Background:/B＞ Endodermal sinus tumor (EST) is one of the germ cell tumor and relatively common tumor that is related with the children. Osteosarcoma is one of the most common secondary malignancy in the children. But secondary osteosarcoma arising after the treatment of EST is rare. We report a case of osteosarcoma found after treatment of sacrococcygeal EST with high dose cisplatin, bleomycin, etoposide (HDPEB) regimen.br＞ B＞Case:/B＞ A 7-year-old girl presented with right thigh and knee joint pain who had been diagnosed sacrococcygeal endodermal sinus tumor 5 years ago and treated with HDPEB regimen. We detected a mass shadow on right distal femur, showing osteosclerotic and permeative osteolytic lesion on X-ray. And the mass showed low to intermediate signal intensity on T1 weighted image and heterogeneous signal intensity on T2 weighted image on MRI. The mass was diagnosed as osteoblastic osteosarcoma on incisional biopsy.br＞ Consequently we started neoadjuvant chemotheraphy with POG-8651 protocol and she received excision & bony structure reconstruction with allo-bone and cement prosthesis. She took a follow-up bone scan and PET-CT and the results revealed no hypermetabolic lesions.br＞ B＞Discussion:/B＞ The survival rate of malignant germ cell tumors improved for last several decades but some problems have occurred, especially secondary malignancies. So the clinician and the survivor who received chemotherapy due to endodermal sinus tumors should keep in mind the possibility of secondary malignancy.