Iron oxide nanoparticles have been synthesized by laser-induced vapor-phase reaction. Mean diameter of the particles varied from approximate 4 nm to 40 nm and the crystalline phase was transited from γ-Fe2O3 to α-Fe2O3 with increasing operation temperature. The nanometer-sized α-Fe2O3 powders were used to fabricate a thick film based gas-sensitive sensor. The effects of the doped noble metals or carbonate and the calcination temperature on sensitivity of the sensor were studied. The results show that the α-Fe2O3 was doped with 3% noble metal or SrCO3 presents the best sensitivity to CO.