Background: Adding nanoparticles to working fluids such as compressor oil can be a solution to ameliorate the efficiency of refrigeration systems. Using the mixture of nanoparticles and oil that is called nanolubricant (or nanofluid) can augment the heat removal in refrigeration systems, however, in the same time, the pumping power will be increased as the viscosity of nanolubricants is higher than that of usual oils. Therefore, the measurement of nanolubricant viscosity is a prerequisite to estimating the pumping power in refrigeration systems.& para;& para;Methods: Experiments section has been divided into three sections. First, the functionalization method of MWCNTs is presented. Then, the preparation of nanolubricants is explained. Finally, the viscosity measurement approach is explained.& para;& para;Results: To increase the dispersibility of MWCNTs in compressor oil, functionalization of MWCNTs was done through attaching -OH and -COOH groups with the aid of 65 wt% nitric acid solution. It was observed that with increasing the viscosity of the base oil, the stability of MWCNTs based nanofluids improves. Then, the viscosity of nanofluids has been measured at a temperature range of 15 degrees C to 50 degrees C. It was found that at 50 degrees C and mass concentration of 0.1%, viscosity enhances between 40 and 90%, depends on the type of base compressor oil.& para;& para;Conclusion: Four correlations involving plane, paraboloid, Gaussian, and Lorentzian functions were suggested for the viscosity of nanolubricants. In the near future, it is expected from the results of this study that refrigeration systems will have included their Freon based refrigerants with nanoparticles.