An increase in the global energy demand and also the shortage of conventional CRUDE OIL resources have led to an increase in the attention to unconventional CRUDE OIL resources. However, unconventional CRUDE OILs need additional processes which make their production and refining costly. Therefore, some techniques including heating, dilution, or creating OIL in water emulsions have been proposed to solve this problem. In the present study, the dilution method has been investigated to reduce the viscosity of heavy IRANIAN CRUDE OIL produced in the Nowrouz field which has a viscosity of 608 mPa. s and API of 19. 5 at 25° C. After a preliminary diluent selection, the examined diluents were kerosene, diesel, and toluene in the range of 5-30% v/v in 25° C. It was found that using toluene at 30% v/v reduces the viscosity of CRUDE OIL/diluent blend up to 97% in comparison with initial CRUDE OIL. To predict the viscosity of CRUDE OIL/diluent blend, simple mixing rules and mixing rules based on the viscosity blending index have been examined. It was found that the prediction accuracy for the CRUDE OIL/toluene blends was not satisfactory. Koval, Maxwell, and Almaamari mixing rules showed the best results for predicting the viscosity of blends using kerosene and diesel diluents.