Carbon nanotubes (CNTs) were synthesized by the fluidized-bed CVD synthesis of acetylene (C2H2) on a magnesium oxide (MgO) powder impregnated with an iron nitrate (Fe(NO3)3.9H2O) solution. The synthesis parameters were selected as: the synthesis temperatures of 500 and 800°C, the iron content in the precursor of 5% and the synthesis time of 30 min. The synthesized material was purified by using HCl and then characterized by transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis. The purified samples which synthesized temperature of 500 and 800°C consist of mostly multi walled and single walled carbon nanotubes, respectively. After synthesis of CNTs, the polyaninine-doped boric acid (PANI-B)/CNT composites were prepared by coagulation method. Fourier transform infrared (FT-IR) were used to characterize the PANI-B/CNTs composites. Thermal stability and glass transition temperature (Tg) were measured by thermogravimetry (TG) and differential scanning calorimetry (DSC), respectively. The TG and derivative thermogravimetry (DTG) curves indicated that with augment of CNTs content, the thermal stability of PANI-B/CNTs composites increased continuously. While, Tg increased and then decreased with the CNTs content increasing from 0 to 20 mass %.