In this research, the effect of carboxy methyl cellulose (CMC) on the rheological properties of two different water-based drilling fluids was studied. It was observed that the fluid loss decreased as CMC concentration increased, but the rate of decrease was too low to notice after 1 gr. CMC. It was also observed that shear stress increased as the CMC concentration increased. Experimentally, it was observed that 1 gr. CMC/350 ml of drilling fluid slurry is sufficiently appropriate for optimum rheological efficiency. The pore throat plugging criteria for the two drilling fluids was studied and conditions leading to perforation, pore plugging, and development of empirical correlations for the plugging criteria was determined.