This work focused on improving the rheological properties and thermal stability of oil-based drilling fluids by using the mixture of organo-montmorillonite (OMt) and organo-sepiolite (OSep) as the rheological additive. OMt and OSep were prepared in water. X-ray diffraction (XRD), scanning electron microscope and transmission electron microscope were applied to characterize the structure of organoclays (OC). The OC/oil gels were characterized using an appropriate XRD method. Dynamic rheological test was used to appraise the rheological behavior, viscosity, gel strength and thixotropy of OC/oil fluids aged at different temperatures. OMt firstly swelled in oil at low temperature, and then exfoliated above 150 °C. OSep maintained its crystal structure all the time. The mixing of these two OC did not obviously influence their structures. However, the gel formation ability was promoted, resulting in improvement of rheological properties and thermal stability of oil-based drilling fluids. The nanolayers of OMt and nanofibers of OSep interweaved with each other, reinforcing the network structure and protected them form collapse at high temperatures. The mixture of OMt and OSep with mass proportion of 50% for each displayed the optimal rheological properties and thermal ability.