Purpose: In this study, we investigated whether extravasation could be predicted by measuring the dose rate once at the injection site before PET/CT scanning. Materials and Methods: The dose rate at the injection site was measured for 412 patients, and the severity of extravasation was evaluated in the F-18 FDG PET/CT images. Then, extravasation activity and %ID were calculated with VOI analysis. Results: The dose rate at the injection site and %ID showed significant differences in the mild and moderate extravasation groups. Dose rates were 15.9±5.8 mR/hr, 18.7±9.9 mR/hr and 57.1±5.8 mR/hr in the no, minor and moderate extravasation groups, respectively. The extravasated %ID in the minor and moderate extravasation groups were 0.022±0.020 and 0.438±0.483. Also, ROC analysis showed good performance of dose rate in distinguishing between mild and moderate extravasation. Conclusion: The dose rate at the injection site can predict moderate or severe extravasation, helping determine arm position during PET/CT scan and whether to conduct additional scans.