Polymers with cyanobiphenyl groups (PCBAn) in their side-chains with different spacer lengths (n=2,6,11) and different molecular weights: (Mw), were prepared. The phase behavior of the polymers and that of liquid-crystalline mixtures (LCMn) consisting of PCBAn and low-molecular-weight liquid crystal, were observed. LCM2 and LCM6 showed liquid-crystalline property of nematic phase, while LCM11 showed liquid-crystalline property of smectic phase. The electrorheological properties of LCMn were investigated. The viscosity of LCMn increased upon application of a DC electric field, and the degree of increase was characteristic to each n value. LCM6 of different Mw showed nearly the same electrorheological behavior except for the LCM6 of lowest Mw, which displayed a large increase in viscosity upon application of the electric field.