  The electronic structure of an isostructural series of molecular conductors,
{\beta}'-X[Pd(dmit)_2]_2 is systematically studied by a first-principles method
based on the density-functional theory. The calculated band structures are
fitted to the tight-binding model based on Pd(dmit)_2 dimers on the triangular
lattice. We find systematic variation in the anisotropy of the transfer
integrals along the three directions of the triangular lattice taking different
values. The transfer integral along the face-to-face stacking direction of
Pd(dmit)_2 dimers is always the largest. Around the quantum spin liquid, X =
EtMe_3Sb, the other two transfer integrals become comparable. We also report
sensible differences in the distribution of wavefunctions near the Fermi level
between the two dmit ligands of the Pd(dmit)_2 molecule.
