Driven by excellent CO oxidation capability of gold nanoparticles M room temperature, a TiO2 supported Au nanoparticles (Au/TiO2) is synthesized by deposition-precipitation with urea (DPU). To solve the sintering issue of gold nanoparticles during activation process, atmospheric-pressure cold O-2 plasma is employed to activate the Au/TiO2 catalysts and compared with the conventional calcination. The plasma activated one (S-P) shows poorer activity compared to the calcined one (S-C). However, S-P with further TPD treatment (S-PT) shows the best catalytic activity among above-mentioned samples. To elucidate the plasma effect, TEM, XPS, UV-vis spectroscopy and in situ DRIFTS techniques are used for catalyst characterization. Based on TEM results, S-P sample has smaller particle size and narrower size distribution compared to S-C sample, while the high content of lowcoordinated Au atoms and the poisoning nitrate species over S-P are revealed by in situ DRIFTS. Upon further TPD treatment, the poisoning species occupying the active sites is desorbed from catalyst surface and the improvements in Au particle size and size distribution are maintained. The mechanism for plasma effect on activation of Au/TiO2 prepared by DPU is also discussed.