We have measured electron-positron momentum density in (TMTSF)(2)ClO4 by the positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique In order to interpret the experimental results, the electronic structure of (TMTSF)(2)ClO4 as well as the positron state therein has been investigated by ab initio theoretical calculations 2D-ACAR spectra have been also calculated and compared with the experimental ones through their anisotropies and Lock-Crisp-West remapped spectra The Fermi-surface-related structure is discussed.