Electrorheological (ER) fluids are suspensions composed of ionomer solid particles dispersed in insulating liquids, which show abrupt changes of theological properties under the stimulation of an external electric field. In this study, electrical and ER properties of end-chain and mid-chain functional macrophotoinitiators of poly(epsilon-caprolactone) were investigated. For this purpose 4 different samples of poly(epsilon-caprolactone) were synthesized with different molar masses. They were then ground milled for a few hours to obtain micron size. The particle sizes of the poly(epsilon-caprolactone)s were determined by dynamic light scattering. Suspensions of poly(epsilon-caprolactone)s were prepared in silicone oil (SO), at a series of concentrations (c = 5-20%, m/m). The gravitational stability of suspensions against sedimentation was determined at constant temperature (T = 25 degrees C). Flow times of the suspensions were measured under no electric field (E = 0 kV/mm), and under an external applied electric field (E not equal 0 kV/mm) strengths and a strong ER activities were observed for all the poly(epsilon-caprolactone)s/SO suspensions. Further, the effects of suspension concentration, molar mass of the poly(epsilon-caprolactone)s, shear rate, electric field strength, frequency, promoter and temperature onto ER activities of suspensions were investigated.