In presence of nano-In2O3 which were synthesised via a reverse microemulsion, Polyaniline/nano-In2O3 composites were prepared by in-situ polymerization of aniline in 5-sulphosalicylic acid(SSA) aqueous solution. They were characterized by means of TEM, XRD and FTIR.. TEM and XRD showed that the average diameter of In2O3 particles was 15nm with a narrow size distribution and with a high degree of crystallization. The FTIR suggested that the structure of PAn-SSA was not be changed by the mixture of In2O3. Sensitivity of the composites to 100~1000ppm NH3 were studied, the results reveal that polyaniline/nano-In2O3 composites have short response time and good reversibility, the gas sensitive of composites to NH3 under 300ppm increased linearly with the increasing concentration of NH3 and decreased with the increasing of In2O3 concentration. Long-term stability of polyaniline/nano-In2O3 composites were also investagated, it can be concluded that the organic-inorganic hybrid materials have better environmental stability.