In this work, HOMO-LUMO energy gap and the lowest excitation energy of poly [2,7-(9,9-dihexylfluorene)-co-alt-2,5-(decylthiophene) were performed by different methods. The obtained results indicate that TDDFT(B3LYP/6-31G*)//B3LYP/6-31G* calculations can be useful to provide reliable energetic and structural results of this polymer. The HOMO-LUMO predictions were not accurately obtained as compared to the experimental results. The inverse chain length approximation by using TDDFT(B3LYP/6-31G*)//B3LYP/6-31G* calculations provides energy gap of 2.50 eV, which is in an excellent agreement with the experimental data. However, it was found that the HOMO-LUMO energy gaps obtained from B3LYP calculations were still far from the experimental data.