In this research, two groups of experiments were conducted to investigate the effects of silica (SiO2) nanoparticles on the filtration and rheological properties of water-based drilling fluids. In the first group, bentonite, chrome-free lignosulfonate (CFL) and carboxymethyl cellulose (CMC) were used in different concentrations to obtain base fluids. Nanofluids were prepared by adding 0.5 g of four different silica nanoparticles into these drilling fluids. Comparison of rheological properties, fluid loss amounts and mud cake thicknesses were performed at two different temperatures (25 degrees C and 49 degrees C). It was concluded that all of the nanoparticles reduced the filtration efficiency of bentonite muds. On the contrary, a reduction in fluid loss was observed for some lignosulfonate muds containing nanoparticle. However, no significant change in mud cake thickness and rheological properties was seen for both drilling fluids.