Developing drilling fluids with a low solid content is preferred for better solid control, easier fluid maintenance, faster penetration rate, and improved hydraulics in oil and natural gas exploration. In this work, low solid drilling fluids were prepared by replacing a certain amount of bentonite in traditional fluids with a small amount of cellulose nanoparticles (CNPs), including cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), aiming to improve cost-efficiency, eco-friendliness, and safety of the fluids. Rheological and filtration properties of the fluids were investigated and compared with those of the traditional fluids without CNPs. The results indicated that the low solid fluids exhibited an obvious shear thinning behavior. The morphology of CNPs played an essential role in modifying the fluid properties. The fluids with smaller CNCs had lower viscosity, yield point, and gel strength; and the formed filter cakes had a larger porosity and lower permeability, in comparison with the fluids having larger and longer CNFs. These differences were attributed to the distinct CNC and CNF morphologies and their interactions with bentonite. The study helps promote applications of biodegradable and renewable additives in well service fluids for oil and natural gas exploitation and production. (C) 2016 Elsevier B.V. All rights reserved.