Methyl sulfide terminated trans-oligo(p-phenylenevinylene) derivatives (OPVn, n is the number of phenyl rings) were synthesized, and reversible sol-gel transition was observed in a variety of organic solvents. Investigations with UV-vis, fluorescence, and H-1 NMR spectroscopy revealed that aromatic pi-pi stacking and van der Waals forces were important in the formation of the gels, with the former being the main driving force for sol-gel transition. The pi-conjugation length showed a key influence on self-assembly and gelation property: the gel-to-sol transition temperature (T-gel) increased with pi-conjugation length. The gels of OPV4-7 can self-assemble into one-dimensional fibers with different sizes and shapes, depending on their pi-conjugation length. On the basis of X-ray diffraction measurements and spectroscopic data, a self-assembly model was proposed. Our observation may be useful for designing functional pi-gelators based on pi-pi stacking.