Magnesium and its alloys are potential biodegradable materials due to their outstanding biological performance, but their poor corrosion resistance greatly limits their applications as bone implants. This paper investigates the influence of in situ magnesium oxide coating on corrosion resistance of pure magnesium in normal saline. It was discovered by SEM that rough and porous oxide coatings were obtained on the surface of the pure magnesium after heat-treated at 400~500°C and the samples were severely corroded after immersion in 0.9 wt.% NaCl solution for 3 days. It also showed that the weight loss rates of the treated pure magnesium (TPM) samples were about 5~6 times higher than that of the untreated pure magnesium (UPM) sample. Electrochemical measurements showed that the corrosion current density (icorr) of the TPM samples was one order of magnitude higher than that of the UPM sample.