In this study, predictive capabilities of apparent viscosity of oil-based DRILLING FLUIDs which is used in National Iranian South Oilfields Company (NISOC) were evaluated using Newtonian and non-Newtonian models to drive a new suitable equation. The non-Newtonian models include Bingham plastic, Power law, Herschel-Bulkley, Casson, and Robertson-Stiff. To validate the results, the calculated viscosity from rheology models was compared to the fann 35 data of viscometer. The results showed that Robertson-Stiff model has the best prediction of shear stress and viscosity with an absolute average percent error of 3. 58. This was followed by Herschel-Bulkley, Casson, Power law, Bingham plastic, and Newtonian with the absolute average percent error of 3. 68, 3. 77, 9. 04, 20. 09, and 44. 02 respectively. Therefore, the new equation was proposed to predict the shear stress for oil-based DRILLING FLUIDs which is used in Southwestern Iranian Oilfields. In comparison to the results of the experimental data of this study, it was revealed that the proposed equation has a good agreement with the real shear stress and apparent viscosities.