Abstract: Oil soluble BaSO₄@SiO₂-OTMS nanoparticles were prepared by a sol-gel and in-situ surface modification method. The morphology and structure of the as-prepared BaSO₄@SiO₂-OTMS nanoparticles were analyzed by transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectrometry. The thermal stability of BaSO₄@SiO₂-OTMS nanoparticles was evaluated by thermogravimetric analysis, and its effects on the lubrication performance and rheological behavior of an oil-based drilling fluid were investigated with a reciprocating friction and wear tester and a six-speed rotational viscometer. Results show that the as-prepared BaSO₄@SiO₂ nanoparticles consisted of BaSO₄ nano-cores with an average diameter of 30 nm and nano-silica shell with a thickness of about 8 nm. Thanks to the surface-modification of nano-silica by oleophilic octadecyltrimethoxysilane (denoted as OTMS), the as-prepared BaSO₄@SiO₂-OTMS nanoparticles can be well dispersed in liquid paraffin. In the meantime, BaSO₄@SiO₂-OTMS nanoparticles as the additive in the oil-based drilling fluid can reduce the flow resistance, abate the wear of the bit, and increase the drilling speed. When BaSO₄@SiO₂-OTMS nanoparticles were added in the oil-based drilling fluid at a relatively low concentration, the viscosity of the drilling fluid tended to decline with increasing additive content. At an additive concentration of 0.8% (mass fraction), the viscosity of the drilling fluid and the friction coefficient of the steel ball-shale disc sliding pair and the width of wear track of the shale disc reached the minimum.