Abstract: Graphene oxide (GO) prepared by a modified Hummers method was reduced by NaBH₄ at room temperature for 12 h in a 0.5 mg/mL GO water suspension to obtain reduced GO (RGO), using CaCl₂ as a catalyst. The GO and RGO were characterized by XPS, FT-IR, UV-Vis and electrical resistivity measurements. Results show that CaCl₂ improves the reduction ability of NaBH₄ for oxygen-containing functional groups on the GO. After reduction, the C/O atomic ratio increased, most of the oxygen-containing functional groups were eliminated, and the electrical resistance decreased significantly. The highest C/O atomic ratio was 5.38 and the lowest electrical resistance of RGO was 18.6 kΩ/sq, the latter being about two orders of magnitudes lower than the RGO prepared without CaCl₂. This reduction method opens a possibility of reducing GO under ambient conditions without using a toxic/corrosive reducing agent and an organic solvent.