The effects of active sites and valence states were investigated over Pd/TiO2 catalyst on simultaneous oxidation of CH4 and CO. The Pd species (PdO) crystallite size increased with increasing Pd loadings, which results in enhancement of the activity of CH4 oxidation. Different results from the activity of CH4 and CO oxidation were shown to be dependent on the Pd valence state on the surface of the catalyst prepared through a thermal treatment. XRD and H2-TPR analysis confirmed that Pd2+ species was predominated in the calcination catalyst, while Pd0 species was predominated in the reduction catalyst. Additionally, it could be found that the valence state of Pd was a more important factor on the catalytic activity than that of factors as the surface area and pore volume. The reaction mechanism of CH4 and CO followed by the valence state of Pd could be identified using FT-IR analysis.