The device performances including the emission spectrum and device efficiency of blue-emitting beta-phase poly(9,9-di-n-octylfluorene) (beta-PFO) annealed at different temperatures from 40 to 160 degrees C are investigated. The relatively unchanged beta-PFO content along with dominating photoluminescence (PL) and electroluminescence (EL) spectra are observed at the thermal annealing temperature (T-TA) below 90 degrees C. As T-TA > 90 degrees C, a gradual decrease in the beta-PFO fraction is observed, and there is a significant blue shift of emission spectra at T-TA > 130 degrees C which is due to a generation of large amount of crystalline PFO acting as an energy harvester. As for the device performance, beta-PFO can give relatively stable current efficiency and brightness at a temperature between 50 and 80 degrees C. Based on these results, it suggests that blue-emitting beta-PFO is practically applicable in display and illumination as a stable deep-blue light source. (C) 2012 Elsevier Ltd. All rights reserved.