Mixed-phase TiO2 nanopowders with different ratios of anatase and rutile have been successfully synthesized using atmospheric pressure plasma jet driven by dual-frequency power sources. The crystal structures of the TiO2 nanopowders were characterized by X-ray diffraction, SAED, HRTEM, and Raman shift spectroscopy. These results indicated that samples possessed anatase and rutile structure, in addition, the crystallinity of the TiO2 nanopowders increased and the chlorine contamination decreased with discharge RF power increasing. The photocatalytic activity of the TiO2 nanopowders was evaluated by decomposition methylene blue solution. The TiO2 nanopowders which were produced at the discharge RF power of 110 W had the highest photocatalytic activity. Optical emission spectroscopy (OES) was used to detect various excited species in the plasma jet. The results indicate that the various RF power significantly changes the intensities of emission lines (Ar, Ar+, Ti, Ti+, Ti2+, Ti3+ and O), which results in the TiO2 nanopowders a mixture of anatase and rutile phases. The nonequilibrium chemical composition could be formed in one step without anneal. It may have potential applications for synthesizing nanosized particles of high crystallinity by reactive nonthermal plasma processing.