Drilling fluids with bentonite can often cause major problems such as flocculation, loss of rheology control and excessive friction due to high clay concentration. Polymers can mitigate such problems, but all these come with much higher costs. Partially hydrolyzed Polyacrylamide (PHPA) polymers are used as additives to bentonite water based fluids to stabilize shales and in HTHP applications because of better thermal stability (PHPA). It also acts as coating on cuttings preventing agglomeration minimizing fluid loss and aiding cuttings transport. There is not much published information about PHPA optimum concentration and its mode of action and we aim to identifying these and we attempt to explain the effectiveness for achieving better design of drilling fluids. Four bentonite (2%, 3%, 4%, 5%) and five polymer (PHPA) concentrations (0.05%, 0.1%, 0.3%. 0.5%, 0.7%) were used. The rheological behavior was described very well by the Herschel-Bulkley model. PHPA addition resulted in increase of apparent viscosity, large decrease of the yield stress and in great reduction of the fluid loss and models are presented for such behavior. The morphology of the filter cakes was investigated using Scanning Electron Microscopy in order to understand the adsorption of the polymer onto the clay particles. We try to explain the effectiveness of polymer addition and its mode of action, which further enhances our understanding of the polymer-clay interaction. This work aids into formulating better water-based fluids for such applications using optimal polymer concentrations. Better understanding of the mode of interaction of the PHPA additive with clay particles in drilling fluids has been achieved by in-depth analysis of the effect of various concentrations on rheological and filtration properties of drilling fluids. This allows industry to work with optimal PHPA concentrations for more efficient drilling.