The formation rule and mechanism of edge defect and oxidation on MoS2 through thermal treatment in air and nitrogen atmospheres were explored in this work. Atomic force microscope (AFM), X-ray photoelectron spectroscope (XPS), and scanning electron microscope-energy dispersive spectrum (SEM-EDS) demonstrated that both edge defects and surface oxidation occurred on MoS2 when being thermally treated in air atmosphere, while pure defective MoS2 structure formed in N-2 atmosphere. The degrees of defect and oxidation increased with the increase of temperature and time under thermal treatment. Auger electron spectrometer (AES) revealed that the oxidation of MoS2 was limited on the top surface and the mechanism for the oxidation was the substitution of O for the top S layer. AES and SEM-EDS results suggested that the origin of defects on MoS2 in air atmosphere was a combination of MoS2 oxidation and thermal effect, while that in N-2 atmosphere was simply because of the thermal effect. This work indicated that defects and oxidation on MoS2 could be well controlled through the modification of thermal temperature, atmosphere and time, which might give a significant guidance for the better application of MoS2.