A successful drilling operation depends strongly on a useful drilling FLUID system. Using nanoparticles (NP)s to formulate intelligent drilling FLUIDs gives them a wide range of optimal properties under different operating conditions and resolves any operational problems. In this study, in order to provide an effective solution for improving the rheological and high pressure/high temperature (HPHT) filtration properties of the oilbased drilling FLUID (OBDF), copper oxide (CuO) NPs were synthesized with dandelion morphology. CuO dandelions were characterized by using XRD, FTIR, SEM, and zeta potential measurements. A long time stabilized nanoFLUID (NF) was prepared and evaluated. The OBDF samples consisting of various amounts of NF ranging from 1 to 11% (V/V) were prepared. Then, the FLUID LOSS and rheological properties of OBDF were examined. The results showed that the OBDF containing 7% (V/V) NF was appropriate to improve the rheological properties such as yield point (YP), apparent viscosity (AV), and gel strength (GS). In addition, the minimum HPHT filtration value of 2 ml was acquired for the drilling FLUID containing 9% (V/V) NF. In conclusion, CuO NPs demonstrated a positive effect on the performance of the OBDF system.