A method of measuring the viscoelastic properties of an electrorheological fluid (corn starch in mineral oil) using oscillatory flow in a thin rectangular tubular channel is described. The channel is formed by two conducting plates separated by insulators on two edges, and an electric field is established using the plates as electrodes. The tube is attached to a viscoelasticity analyzer that produces the desired oscillatory flow over a wide range of amplitudes at a selected frequency. The amplitude and phase of the pressure drop and volume flow are measured, from which the three fundamental parameters−shear stress, shear rate, and shear strain−are calculated at the channel walls [G. B. Thurston, J. Appl. Phys.30, 1885–1860 (1959)]. Mineral oil containing starch at concentrations from 1 to 10 g % were measured at 2 Hz with field strengths from 0 to 1829 V/mm. The shear rate dependence of the viscosity and elasticity vary with the electric field strength. The microstructure yields at a stress level which is identified in the elastic component of the shear stress. Plots of the viscous and elastic stress vs strain show that at low field strengths the microstructure degrades at strains<0.1, but at high field strengths, it degrades near unit strain. With higher starch concentrations, viscoelastic dilatancy is evident at high field strengths and high strains.