As spin-coating polyoctylfluorene (PFO) thin films were thermally annealed at various temperatures and their photoluminescence, electroluminescence and crystalline nanostructure was characterized, alpha phase PFO generate when annealing temperature reaches 80 degrees C and the performance of corresponding electroluminescence device was dramatically enhanced due to the self-doping effect. As the annealing temperature increases, the photoluminescence and electroluminescence decrease. Both in out-of-plane and in-plane grazing incident X-ray diffraction pattern the PFO thin film annealed at higher temperature has stronger diffraction peaks, indicating higher degree of crystallinity. The formation of vacancy defects and increase of interface roughness in crystalline process is considered as the reason for the reduction of luminescence. These results show that the electro-optical properties of PFO can be enhanced through controlling annealing temperature to form alpha phase PFO in low degree of crystallinity. (C) 2012 Elsevier Ltd. All rights reserved.