A successful drilling operation requires an effective drilling fluid system. The aim of this work is to provide an effective solution for improving the rheological and filtration properties of water-based drilling fluid by using CuO nanofluid additive. CuO nanoparticles were synthesized by hydrothermal method using autoclave, which can control the temperature as well as pressure. Then CuO nanofluid (eco-friendly ethylene glycol based) were produced to use as a drilling fluid additive. X-ray diffraction, Fourier-transformed infrared, scanning electron microscope were used to characterize nanoparticles. The results confirmed clearly the formation of high purity CuO nanoparticles forming a wire shape structure. The operating parameters were optimized by experimental design method and based on the optimal results, two long time stabilized nanofluids were prepared to improve the rheological properties and the fluid loss of a polymeric water-based drilling fluid. Xanthan, polyanionic cellulose and starch are commonly used in drilling fluids to improve rheological and fluid loss properties. Also, the effect of pH level of nanofluids on the improvement of water-based drilling fluid properties was investigated. The results showed that the nanofluid with pH=8 can be used as the best additive to improve the drilling fluid properties. The improvement of the yield point, apparent viscosity, 10-second and 10-minute gel strengths of the drilling fluid as well as the fluid loss were 45, 33, 200, 100 and 44 %, respectively.