Due to its similar electrochemistry to that of the lithium (Li)-ion battery but the high abundance of sodium (Na) in the earth, Na-ion battery is expected to be a promising alternative of the former, especially in stationary energy storage applications. Although it has been well-known that the Na-ions can be co-intercalated in graphite in some ether-based electrolytes, a number of important questions related to the Na-ion intercalation remain unclear. In order to better understand the sodium intercalation in graphite, Na-ions were intercalated in highly oriented pyrolytic graphite (HOPG) by immersing the latter in a sodium-biphenyl-monoglyme (Na-BP-DME) solution. The staging behavior that often occurs during Li-ion intercalation in graphite was observed in the Na-intercalated HOPG, on the basis of X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) characterization. The chemical formula of the DME-Na co-intercalated HOPG was determined to be Na(DME)2C26 by a combination of Fourier-transformed infrared (FTIR) and Raman spectroscopy, thermogravimetry and the above structural characterizing techniques. The negligible impact of co-intercalation on the structural integrity of HOPG demonstrates that HOPG can be a promising anode material of the Na-ion batteries. The lower coordination number between the Na+ ions and DME and the interaction between DME and the graphite layers are believed to be responsible for the structural stability of the co-intercalated graphite. Finally the chemical stability of the ternary graphite intercalation compounds in air was evaluated.