This novel research, aimed to improve the properties of Na-bentonite water based drilling mud (WBDM) and reduce its cost by adding multi-walled carbon nanotubes functionalized with copper oxide nanoparticles (CuONPs/MWCNTs). For this purpose, fistly, copper oxide nanoparticles (CuONPs) and multi-walled carbon nanotubes (MWCNTs) were synthesized separately, then MWCNTs were functionalized with CuONPs by chemical vapor deposition (CVD) method. Secondly, CuONPs/MWCNTs were characterized by X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). Thirdly, CuONPs/ MWCNTs nanocomposite was added to the WBDM at different rates (0.001 to 0.01% w/v). Finally, rheological, filtration loss and cake thickness property analyzes were performed according to API standarts. As a result of this study, a significant improvement was observed in the rheological and filtration loss properties of WBDM with the addition of small amounts of CuONPs/MWCNTs. In particular, plastic viscosity (PV), apparent viscosity (AV), yield point (YP), 10 s and 10 min values increased by 57%, 36.7%, 28%, 33.3% and 44.4% respectively, the value of fitration loss decreased by 13.8%. With this improvement will increase the lifetime of the drilling mud is determined and its costs will decrease.