Abstarct: Abstract The success of any well-drilling operation depends on many factors and one of the most important is the drilling fluid. Drilling fluids cool down and lubricate the drill bit. remove cuttings from the hole. prevent formation damage. suspend cuttings and weighting materials when circulation is stopped. and cake off the permeable formation by retarding the passage of fluid into the formation. However. drilling operations face great technical challenges with rheology and filtration and drilling fluid loss being the most notable of them. These problems In this study. the effect of adding montmorillonite clay nanoparticles with CMC carboxymethyl cellulose polymer in water-baxsed drilling mud to control filtration and rheology during drilling has been investigated. At first. the problems caused by drilling in different laxyers such as shale laxyers. as well as the mechanism of mud filtration in various formations and clay minerals in mud have been investigated. After that. the rheology and filtration tests of water-baxsed drilling mud containing polymer and nanoparticles were performed. The results show that the presence of nanoparticles has caused a slight increase in the rheological property of the drilling mud. and at a maximum concentration of 1 percent by weight. it has shown a tangible increase in the viscosity property up to 100 percent. The results show that the addition of composite to the water baxse mud causes a large increase in viscosity and gel strength of the drilling mud. The results also show that the gel strength properties of drilling mud decrease in low concentrations. which is due to the interference of the surface charges of the clay plates. Finally. the results of filtration showed that the use of this composite shows a greater ability to reduce the filtration rate than the use of polymers alone. and in general. the amount of mud filtration drop is significantly reduced in low temperature-pressure mode from 26 CM2 to about 4.5 CM2.