This research is to synthesize PANI (polyaniline) thin film for polymer organic solar cells as a hole transport layer on the top of ITO substrate by using electrochemical (cyclic voltammetry) method. The device structure is ITO (150 nm) / PANI (50 nm) / P3HT: PCBM (100 nm) / Al (200 nm). We investigated surface morphology, conductivity, and light transmission of the PANI thin film from different aniline monomer concentration and studied the factors on device efficiency, also compared with the device structured with hole transport layer PEDOT:PSS. In this study, we found PANI thin films synthesized with different aniline monomer concentration, their light transmission over 80% at the range of 450 nm ~ 650nm wavelength and the conductivity up to 0.6 S/cm. It shows that PANI thin film suitably act as hole transport layer. In addition, we found morphology of PANI thin film that varied with different aniline monomer concentration. The power conversion efficiency of the device mainly affected by morphology with different aniline monomer concentration. Comparing to other parameters of concentration, the 0.3M aniline monomer concentration polymerized PANI thin film owned the most appropriate surface morphology, and the power conversion efficiency up to 1.76%.