The thermal stability of oil-based drilling fluids which contain organo-montmorillonite (OMt) is significant for drilling operations. Either thermal degradation or desorption of interlayer surfactants may result in the decline of rheological properties of oil-based drilling fluids. Aiming to confirm the definite reason, the structures, rheological properties, and elemental analysis of different OMt samples in oil-based drilling fluids were characterized in this work. OMt samples were prepared by reacting montmorillonite with different types of cationic surfactants in aqueous solution. colloidal and rheological properties of OMt/oil fluids aged at different temperatures have been investigated. X-ray diffraction, thermal analysis, and elemental testing were employed to reveal the structure of OMt powder and OMt/oil samples. The basal spacing of OMt in oil varied as swelling, exfoliation, and decline from 66 to 250 degrees C. Both desorption and degradation influenced the thermal stability of OMt/oil fluids. Surfactants with short alkyl chains desorbed in oil even at low temperatures, while organic modifiers having long alkyl chains did not. Quaternary ammonium salts degraded into olefins and amines before 200 degrees C, and continuous oxidation occurred at higher temperatures. The resulting olefins and amines first stayed in the interlayer space and then gradually dissolved into the base oil at higher temperatures.