In this work, the forming condition and mechanism of the beta conformation in poly (9,9-dioctylfluorene) (PFO) solution are studied. We find the forming condition of beta conformation is strongly associated with the proportion of the poor solvent (ethanol) in mixed solution (toluene/ethanol). When the proportion of poor solvent is less than 40% (volume ratio), there is no beta conformation in solution; but if the proportion of poor solvent increases to 40%, the beta conformation starts to appear. Static and Dynamic Light Scattering research reveal that the molecular chains become collapsed; meanwhile the degree of aggregation is also enhanced when the ethanol proportion increases and proved by TEM. By changing the PFO concentration, we find the concentrations have great influence on the chains aggregation but little influence on the content of beta conformation. So we infer the beta conformation was formed not in solution but in aggregation, the forming reason was proved to be both intrachain interaction in single chain itself as well as interchain intraction with neighbors in aggregation, which were all enhanced by adding ethanol proportion. The study has a potential meaning to make optoelectronic films whose the condensed state structure was mostly decided by the precursor solution with high charge carrier mobility and devices efficiency by controlling the content of beta conformation in PFO precursor solution. (C) 2016 Elsevier Ltd. All rights reserved.