Detection of flammable and exhaust gases is one of the important subject both in energy-saving and environmental protection endeavors. Among a large group of existing chemical sensors, TiO2 has showed the excellent sensing properties for various gases, such as CO, NOx, CH3CH2OH, H2 and O2. It is well known that TiO2 has three different types of crystal structure, anatase, rutile, and brookite. Anatase TiO2 with a tetragonal structure is one of the most important semiconductors used for the various applications like photovoltaic cell, photocatalysis, and sensors. However, the application of the anatase TiO2 for high temperature gas sensor has been limited because the anatase phase is easily transformed into rutile phase over 600℃. On the other hand, in the case of Au/TiO2 core-shell structure nanoparticle (NPs), it has been found that the anatase structure of TiO2 composing a shell part can be protected up to 800℃ because gold NPs as a core material plays an important role to suppress the transformation of anatase into rutile phase. Furthermore, gold NPs can act as a catalyst on sensing CO gas. In the present work, gold NPs were prepared by colloidal method from HAuCl4·4H2O with sodium citrate as a stabilizing agent while ascorbic acid was used as a reducing agent. Au/TiO2 core-shell NPs were synthesized by normal hydrothermal (NH) method and microwave-assisted hydrothermal (MH) method from TiF4 aqueous solution as a precursor and investigated their CO gas sensing property. The structural analysis of has been carried out by X-ray diffraction (XRD) and transmission electron microscope (TEM). TEM images showed the formation of Au/TiO2 core-shell structured NPs with the diameter from 170 to 210 nm. The size of the core (Au) was ranged from 40 to 50nm while the shell (TiO2) thickness was changed from 60 to 80nm. The Au/TiO2 core-shell structured NPs has been calcined at 600 and 700℃. And then the CO gas sensing properties were measured at 500 and 600℃ under various CO concentration. To fabricate CO gas sensors operated at high temperature with high sensitivity at high temperatures, Au/TiO2 core-shell NPs were synthesized by NH method at 180℃ for 48 hours and MH method at 180℃ for 1 hour. The Au/TiO2 core-shell NPs synthesized by NH method at 180℃ for 48 hours showed the excellent response for CO gas. And then, the Au/TiO2 core-shell NPs were also synthesized by MH method at various conditions such as synthesis temperatures, times, and increasing times. The Au/TiO2 core-shell NPs synthesized at 180℃ showed the highest response for CO as compared to others synthesized in the range from 100℃ to 180℃. In case of synthesis time Au/TiO2 core-shell NPs synthesized for 15 hours showed the highest response for CO as compared to others synthesized in the range from 1 to 24 hours. The Au/TiO2 core-shell NPs showed higher response for CO than that for Au/TiO2 core-shell NPs synthesized by NH method. By using microwave assisted hydrothermal method, the response for CO showed the highest value when synthesis time of Au/TiO2 core-shell NPs was 15 hours.