Three kinds of side-chain liquid crystal polymers (SCLCPs) were prepared in which the lengths of the flexible aliphatic spacer were 6, 8, and 12 methylene units. The SCLCPs consisted of a polymethacrylate backbone carrying cyanobiphenyl mesogenic groups as side chains. All the polymers exhibited liquid crystalline behavior of smectic A phase. The glass transition temperature decreased with increasing aliphatic spacer length. Electrorheological (ER) suspensions were prepared by dispersing the SCLCP particles in silicone oil and their ER effects were investigated. The ER response increased with increasing aliphatic spacer length, indicating that the spacer length between the polymethacrylate backbone and polar mesogenic groups influenced the ER behavior.