The electrorheological fluid (ERF) with coke-like particles from thermal conversion of fluid catalytic cracking (FCC) slurry was prepared and their ER performance was investigated. The results show that an ERF containing these solids is a high performance material with high shear stress, low current density and long-term stability. Shear stress higher than 2 kPa and current density less than 60 muA/cm(2) was obtained in a 2 kV/mm DC field. The volume fraction of clear liquid released from the ERF was less than 5 vol% after several months of static test, indicating the ERF is stable and meets commercial application requirements. The influence of thermal conversion level and the orientation of condensed aromatics on ERF performance are discussed. (C) 2004 Elsevier Ltd. All fights reserved.