Human salivary gland cells (HSG) were irradiated with carbon ion beams supplied by the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan at a low dose rate of 0.5 Gy/h. Clonogenic survival effects of HSG cells after exposure to carbon beams with three different dose-averaged LETs were obtained by means of standard colony-forming assay. Compared to the previous surviving data acquired for carbon beams with similar dose-averaged LETs at therapeutic dose rates (1~5 Gy/min), HSG cells show an obvious dose-rate sparing effect. To derive relative biological effectiveness (RBE) of the carbon beams for HSG cells under the same dose-rate condition, the dose surviving effect of HSG cells irradiated with 60Co γ-rays was measured as well at the dose rate of 0.5 Gy/h. The RBE values obtained at the low dose rate are totally reduced in contrast with those determined at the preceding therapeutic dose rates. The dose-rate sparing effect and the reduction in RBE at the low dose rate imply that the radiation-induced damage by high-LET carbon ion beams at low dose rates could be repaired to a great extent. Based on the important findings, the essential reason of cell lethality by radiation was discussed.