In this study, the viscosifiers for diesel oil drilling fluid were prepared using Thnisian clay modified with cationic surfactant for five concentration levels from 0.5 to 4.0 times the cationic exchange of purified clay (CEC). The organo-modified clay samples were characterized by X-ray diffraction (XRD), Infrared spectroscopy (FTIR), scanning electron micrography (SEM) and specific surface area (BET model). Oil clays suspensions were investigated using polarized light optical microscopy (PLOM) and double cone rheometer. The influence of surfactant concentration on rheological properties of suspensions is related to the different microstructures development which depend on surfactant arrangement in the interlayer of clay. The results showed the growth of yield stress and consistency coefficient as the surfactant concentration is increased and the best behaviors were observed for the 3.0 CEC concentration. This sample was tested for two weight percents (5 and 10 wt%), the higher Herschel Bulldey parameters (tau(0) = 3.6 Pa, K =0.397 and n = 0.85) were found for the organoclay at 10 wt%. The latest sample has the high thixotropy loop area so can be used as viscosity enhancer for the oil drilling fluid. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.