As a potential candidate for the development of anhydrous electrorheological (ER) materials, phosphate cellulose particles were synthesized from a phosphoric ester reaction between cellulose particles and a 2 M phosphoric acid-urea mixture, and then dispersed in silicone oil to prepare the phosphate cellulose-based ER fluid. Rheological measurements were carried out via a rotational rheometer with a high voltage generator in both steady and oscillatory shear modes to investigate the effects of electric field strength and particle concentration on ER performance. The results show not only that the ER properties are enhanced by increasing the particle concentration and electric field strength, but also the cellulose-based ER fluids exhibit viscoelastic behavior under an applied electric field due to the chain formation induced by electric polarization between particles. (C) 2001 Published by Elsevier Science Ltd.