We have performed X-ray diffraction study under pressure for isostructural anion radical salts beta'-Et(2)Me(2)Z[Pd(dmit)(2)](2) (Z=P, As, Sb) at room temperature. The unit cells remained monoclinic and no satellite reflection was detected up to 18 kbar. For the high-pressure superconductor Et2Me2P salt, high-pressure crystal structure analyses were carried out. Obtained band parameters indicate an increase of bandwidth and a decrease of the dimensionality of a HOMO-based band. This is related to a pressure-induced metallic state and a non-metallic state in the higher pressure region.