  We report on the result of quantum Monte Carlo simulation of
quasi-one-dimensional electron systems at 1/4-filling, considering organic
superconductors such as TMTSF- and TMTTF-salts. We focus on the effect of
dimensionality (interchain coupling) on superconducting fluctuation. First we
consider Hubbard model which includes only on-site repulsion U. We find that
the increase of interchain coupling enhances superconducting susceptibility,
although it deforms the nested Fermi surface and suppresses the spin
susceptibility. Next we consider an extended Hubbard model which includes
nearest-neighbor repulsion V . In this case we consider the competition between
different symmetries of electron pairing, d_{x^2-y^2} - and d_xy-wave symmetry,
and find that the particle-particle interaction vertex for d_{x^2-y^2}-wave
pairing is no longer attractive even in the region where nesting condition
still holds. On the other hand, the interaction vertex for d_xy-wave pairing
persists to be attractive. The obtained results for the Hubbard model show
qualitative agreement with recent renormalization group study, and have the
possibility of accounting for the recent experimental result of (TMTTF)2SbF6,
which exhibits a wider superconducting phase than the previous studies.
