Abstract: The composite materials of PANI/CNT were prepared by the in-situ polymerization. Transmitting electronic microscopy (TEM), ultraviolet visible spectrum (UV-VIS), Fourier transfer infrared spectrum (FTIR), thermal gravimetry analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the structures and properties of the composites. The results indicate that aniline (ANI) is preferentially polymerized on the surface of CNT to form a layer of polyaniline (PANI) on the CNT surface and the thickness of the layer increases with increasing ANI content in the reactants. An even PANI layer is built in case of lower ANI content in the reactants while an uneven layer with a few PANI particles adhered is built in higher ANI content. There is no chemical interaction between PANI and CNT. The PANI/CNT composites exhibit higher conductivity and excellent thermal properties and the thermal properties depend also on the loadings of PANI in the composites.