Novel biorenewable castor oil-based polyurethane (PU) thermoset has been prepared via environmentally friendly aqueous dispersion with no organic solvent. Thin film of castor oil-based PU was obtained via solvent casting method of the aqueous dispersion. Dielectric spectroscopy of the PU thin film was investigated at different temperatures and frequencies using dielectric spectrometer from Novocontrol. The dielectric spectra of the castor oil-based PU showed α-, β-, and γ-relaxation processes and ionic conductivity at different temperatures and frequencies. The broadband dielectric spectra of castor oil-based PU was resolved into different relaxation processes based on Havriliak-Negami equation. The VFT equation was used to describe the molecular dynamics of the α-relaxation process. However, on the other hand Arrhenius equation was found to fit the linear activation curves of β- and γ-relaxation processes with apparent activation energies of 68 and 30 kJ/mol, respectively.