The phase transition behavior of liquid crystalline cyclic-siloxanes containing fluorine atoms in the terminal mesogenic groups is reported. The series has different spacer length and the number of fluorine atoms that are included in the terminal mesogenic group. The thermal property of the compounds was investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The influence of the spacer length between cyclic-siloxane core and the terminal mesogenic groups on the mesomorphic properties was discussed. Moreover, we measured rheological property of these compounds in the presence of electric field. We described their electro-rheological effect (ER effect) in consideration of the influence of molecular structure. The cyclic compounds exhibited considerably large ER effect of which the response time to an electric field was fast. They were expected to be used as novel ER fluids in engineering field. (c) 2007 Wiley Periodicals, Inc.