This study evaluated the accuracy of T1-weighted magnetic resonance imaging (MRI) in determining the osteotomy plane in 21 patients with osteosarcoma undergoing limb-salvage surgery. Twelve cases involved the distal femur and 9 cases involved the proximal tibia. Mean patient age was 16.3 years (range, 12–24 years). None of the patients presented with evidence of metastasis. After being placed on neoadjuvant chemotherapy, all patients were treated with en bloc resection and a custom prosthesis. Intramedullary extension was measured on preoperative MRI and radiographs, and also on postoperative specimen by gross and histopathological evaluation. The osteotomy plane was confirmed at 30 mm distal from the primary tumor based on T1-weighted MRI. Mean intramedullary extension measured on MRI, radiographs, and gross examination were 107.4 ± 34.5, 78.6 ± 25.6, and 92.6 ± 20.5 mm, respectively; actual mean extension was 104.3 ± 32.8 mm. No significant differences were noted between the actual extension and the extension measured on MRI according to statistical analysis. Intramedullary extension was measured accurately on MRI, which also confirmed the surgical margins. These findings indicate using 30 mm distal from the primary tumor as the osteotomy plane based on T1-weighted MRI is safe.