This work focus on optical band gap of nanocomposited MEH-PPV:CNTs thin film. In this research we investigate the behavior of optical band gap when the composition of CNTs powder is increased which directly influence the thickness of the thin film. The experimental process entailed for the black powder of CNTs to be first annealed at 450 °C before mixing it with the polymer solution to ensure that the impurities in the CNTs are all removed. The 20 mg MEH-PPV polymer was dissolved in an aromatic solvent which is toluene with a concentration of 1:1. The composite materials were prepared by adding appropriate amounts of CNTs powder (1, 2, 3 and 4 wt%) into the polymer solution to make various ratios of CNTs powder/polymer composites. The optical properties of the thin film were analyzed by using Perkin Elmer Lambda 750 UV/Vis Spectrometer. Thickness of the thin film is measured using Surface profiler Veeco Dektak 150. In this paper, the optical band gap energy is derived by assuming a direct transition of electron between the edge of the valence band and the conduction band. Our results demonstrates that as the thickness decrease from 62, 60, 59, 58 and 57 nm, the optical band gap showed slight decrement from 2.07, 2.07, 2.05, 2.01 and 2.00 eV for respectively.