We probed the gelation and its effect on the photophysical property of a conjugated semiconducting polymer, poly(9,9-dioctylfluorene-2,7-diyl) (PF8), in toluene. The conformational structure characterized by the radius of gyration and the persistence length of PF8 in the dilute toluene solution depended strongly on temperature, where the conjugated chains exhibited a more extended conformation as the temperature was raised. The interchain interaction above the overlap concentration induced aggregation of PF8 to form network clusters in the solution. Gelation took place when the semidilute PF8 solutions were cooled and subsequently aged at -20 degrees C. The gel composed of the dynamically arrested PF8-enriched phase and the isotropic phase due to the occurrence of a macrophase separation at the subambient temperature. The PF8-enriched phase was mesomorphic, consisting of sheetlike aggregates or membranes. A fraction of the PF8 chains or segments within these sheetlike aggregates formed the P-phase which dominated the photoluminescence of the gel. The gel structure Could be disrupted by heating to ca. 45 degrees C, above which the corresponding PL spectra displayed a blue shift because of reduced amount of the beta-phase.