Conducting polyaniline/Cobaltosic oxide (PANI/Co3O4) composites were synthesized for the first time, by in situ deposition technique in the presence of hydrochloric acid (HCl) as a dopant by adding the fine grade powder (an average particle size of approximately 80 nm) of Co3O4 into the polymerization reaction mixture of aniline. The composites obtained were characterized by infrared spectra (IR) and X-ray diffraction (XRD). The composition and the thermal stability of the composites were investigated by TG-DTG. The results suggest that the thermal stability of the composites is higher than that of the pure PANI. The improvement in the thermal stability for the composites is attributed to the interaction between PANI and nano-Co3O4.