In this work, we established the relationship between the thermally induced microstructure development and the photoluminescence efficiency (E-p) of the poly(9,9-dioctylfluorene) (PF8) fibers prepared from the semidilute PF8/chloroform solutions by direct electrospinning. The as-spun (AS) fibers were found to be composed of an amorphous phase and a significant fraction of mesomorphic beta-phase, which may promote the E-p of PF8. The temperature-dependent wide-angle X-ray scattering (WAXS) measurement revealed that the PF8 fibers exhibited complex structural transformation on heating from the AS state, and there existed a strong correlation between E-p and the microstructure developed. The AS fibers exhibited the highest E-p value. The E-p decreased progressively on heating at temperatures below the cold crystallization temperature (T-cc), where the microstructure remained unperturbed. The beta-phase transformed to gamma- or alpha'-phase (C-gamma-/C-alpha'-crystals) above T-cc, and such a structural transformation led to a reduction of E-p. With further heating, the C-gamma-/C-alpha'-crystals transformed into the more stable C-alpha-crystals, presumably as the premelting-reorganization-remelting proceeded at the temperatures over the crystal transformation temperature (T-ct). The crystal transformation process reduced the E-p markedly. Finally, above the melting temperature (T-m), the C-alpha-crystals were disrupted into the disordered molten state, and the E-p value remained largely unchanged up to 200 degrees C.