The association structures of 4-n-pentyl-4'-cyanobiphenyl (5CB) in the solution, isotropic liquid, and nematic liquid crystal states have been studied by infrared electroabsorption spectroscopy. In a 0.5 mol dm-3 carbon tetrachloride solution, the determined value of dipole moment of 5CB (4.9D) agrees excellently with a literature value (4.85D) for monomer, indicating that the 5CB molecule exist as a monomer. Monomeric response of the 5CB molecule to the external field has also been found for the isotropic liquid state. For the nematic phase, marked deviation from the monomeric response has been found, showing the domain formation of the 5CB molecules. The size dependence of the domain response to the applied electric field has also been elucidated.