In this research, three different water-based drilling fluids were prepared according to American Petroleum Institute (API) standards, and effects of walnut shells on the rheological properties of the samples were studied. The walnut shells of 2-, 4-, and 6-mm samples and their equal weight mixtures of 2-4 mm and 4 6 mm were added to the drilling fluid samples in different concentrations. The aim of this study is to determine the optimum walnut shell size, concentration and polymer concentration to minimize the fluid loss during drilling operations. It was observed that polymer (XT) has the optimum effects on the water-based drilling fluid. The optimum concentration of polymer (XT) with the 6-mm walnut shell is determined as 0.4 g/350 ml.