Objective To compare the dosimetric difference of stereotactic body radiation therapy (SBRT) plans for spine metastasis using 6 MV X-ray flattening filter free (FFF) and flattening filter (FF) modes.Methods A total of nine previously treated patients with a total of twelve spine metastasis were retrospectively included and replanned using FFF-SBRT and FF-SBRT, respectively.The dose-volume histograms (DVH), target conformity index (CI), monitor unites (MUs) and treatment time of target volume, organs at risk and normal tissues were compared between the plans.Results Both FFF-SBRT and FF-SBRT met the clinical objectives.Dose distribution of target volume, organs at risk and normal tissues were similar.MUs of the FFF-SBRT were significantly higher than that of FF-SBRT (t =-5.20, P ＜0.0l), while the treatment time was almost half of the latter (t =17.27, P ＜ 0.01).Conclusions Two plan modes are both clinically acceptable.FFF-SBRT plans are delivered in less time and with better efficiency.Trial registration Chinese clinical trial registry, ChiCTR-TRC-14004281.