Nanosized　particle　materials　with　different　structures　and　grain　sizes　were　prepared　by　doping　TiO2　or　ZrO2　with　rare　earth　(RE)　elements,　and　by　changing　the　method　of　treatment.　Their　microstructures　were　confirmed　by　X-ray　diffraction　(XRD)　analyses　and　measurements　of　the　surface　areas,　pore　volumes,　and　pore　sizes　of　the　particles.　The　electrorheological　(ER)　performance　and　dielectric　properties　of　these　materials,　and　the　relationship　between　ER　activity,　microstructure,　and　grain　size　of　these　particle　materials　were　investigated.　The　results　have　shown　that　RE　doping　can　either　increase　or　decrease　the　ER　activity　of　a　material,　which　is　related　to　the　pore　volume　in　the　grain.　Body-centered　tetragonal　TiO2　and　tetragonal　ZrO2　possess　higher　ER　activity　than　tetragonal　TiO　2　and　monoclinic　ZrO2,　respectively.　The　effect　of　grain　size　on　ER　performance　should　not　be　neglected　for　different　materials　in　a　system　with　identical　crystal　structure　and　composition,　and　the　co-action　of　both　larger　grain　size　and　larger　pore　volume　can　play　a　very　important　role　in　nanosized　particle　materials.　Optimal　matching　between　appropriate　RE-doping,　microstructure　and　particle　size,　which　can　be　achieved　by　fine-tuning　the　production　process　of　a　material,　may　provide　a　basis　for　producing　an　ER　material　with　high　activity.