Objective To compare the PET/CT and CT images of lower extremity long bone osteosarcoma, and to explore their performance in detecting distant/local metastases of osteosarcoma and in clinical staging of the lesions. Methods Fourteen patients with biopsy-confirmed lower extremity long bone osteosarcoma were enrolled in the present study. All patients underwent CT and PET/CT examination before operation. The image characteristic of tumors and bone marrow, lung metastases loci were analyzed. Results The mean maximum standard uptake value (SUV_max) of the primary lesions in this group was 10.68 (ranging 3.7-21.1). PET/CT found that the mean SUV_max value in metastasis group was significantly higher than that in non-metastasis group (11.94 vs 8.01, P<0.05). PET/CT also found 6 cases of bone marrow cavity metastases, and CT only found 2 cases. PET/CT detected 3 cases of lung metastases, and CT only found 2 cases. The accuracy of PET/CT in detecting the bone marrow metastases of osteosarcoma was significantly higher than that of CT (P=0.088 67<0.1). PET/CT showed no significant difference in detecting lung metastases compared with CT. Compared with CT, PET/CT increased the TNM stage of 4 patients (P<0.05). Conclusion Compared with CT, PET/CT has a better performance in demonstrating the tumor involvement, in detecting bone marrow metastasis of osteosarcoma, and in tumor staging. The SUV_max value can help to predict the prognosis of patients with osteosarcoma.