The effects of hydrostatic pressure on a structure and the corresponding resonant x-ray scattering (RXS) spectrum have been investigated for the hole-orbital-ordered compound (C2H5NH3)(2)CuCl4. We have found a structural phase transition at P-c similar to 4 GPa, as suggested by the Raman scattering measurement, by observing a peak splitting resulting from an orthorhombic-to-monoclinic symmetry breaking. The reduction of the Jahn-Teller distortion (JTD) toward P-c is also ascertained by structural analyses. The gradual change in the color of the crystal is also observed in connection with the structural change near P-c. The red-colored transparency of the crystal indicates that the system is still insulative above P-c. The observed RXS and fluorescence spectrum at ambient pressure were precisely analyzed. We experimentally confirmed that the RXS intensity comes from the polarization of the density of states of p(x) and p(y) symmetries, which is mainly dominated not by the on-site 3d-4p Coulomb interaction but by the JTD of the CuCl6 octahedron. charged Cl ions neighboring the Cu site. It is also expected that the RXS intensity is proportional to JTD; however, our RXS study under a high pressure shows no striking change as JTD is suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above P-c. The results indicate that the local environment of the Cu2+ ion markedly changes. Two possible structures of EA(2)CuCl(4) above P-c are proposed.