The dielectric properties were studied of a completely cyanoethylated derivative (CN-DPMA) of poly(2,3-dihydroxypropyl methacrylate), which was synthesized by radical polymerization of the monomer with protected hydroxyl groups, followed by deprotection. The experimental results were compared with those already reported for a semiflexible liquid-crystalline polymer with cyanoethyl groups, cyanoethylated O-(2,3-dihydroxypropyl)cellulose (CN-DHPC). By the introduction of two cyano groups on the monomer unit, CN-DPMA shows a high dielectric permittivity of 25-26 at 50 degrees C and 1 kHz, which is comparable to the value of CN-DHPC. A study on the temperature dependence revealed two relaxation modes, one being the beta-relaxation due to local motions in the side chain and the other being the a-relaxation due to the main-chain motions. The beta-relaxation appeared in a temperature region similar to that for CN-DHPC. The fact that only one mode assigned to main-chain motions was detectable for CN-DPMA forms an interesting contrast to CN-DHPC, which shows two main-chain relaxation modes. This difference is attributed to the difference in the backbone flexibility of the two polymers