Nanocomposite comprising Poly(9,9-dioctylfluorene-alt-thiophene) (PDOFT) and graphene sheets was synthesized for the purpose of investigating its enhanced optoelectronic performance caused by the graphene. UV/Vis and photoluminescence spectra of PDOFT/graphene nanocomposite show that the existence of graphene does not alter the optical characteristics wavelength of PDOFT, and both solution samples and thin films emit green light. Meanwhile, the insignificant change in PL quantum efficiency indicates that graphene does not have significant effects on the transfer of excitation energy, the occurrence of self-quenching and the formation of excimer. However, the electric conductivity increases with an increase in the amount of graphene. The ionization potential(HOMO) and the electron affinity(LUMO) measured from the cyclic voltammetry lead to the determination of the optical band gap (Eg(chem)). When used to fabricate an optoelectronic device, the threshold voltage decreases with an increase in the graphene content until an excessive amount of graphene causes an unbalance on the electron and hole mobilities. The device fabricated with PDOFT/graphene with a 5% graphene content has a maximum luminescence of 5908 cd/m2 at a voltage of 9.5 V. The corresponding power efficiency at this maximum luminescence is 0.38 cd/A which is much higher than the device fabricated with pristine PDOFT. From the Admittance spectroscopy of the device, the electron mobility has been proved to increase with the graphene content.