The behavior of the ammonia (NH3) oxidation was by selective catalytic oxidation (SCO) over a nanosized Pt-Rh composite catalyst in a tubular fixed-bed flow quartz reactor (TFBR) at temperatures between 423 and 673 K. The catalysts surface properties were characterized using UV-Vis and TEM. The experimental results show high activities for NH3 removal was achieved during catalytic oxidation over the Pt-Rh catalyst at 673 K with an oxygen content of 4%. N2 was the main product in the NH3-SCO process over the nanosized Pt-Rh composite catalyst. These results also verify that the Pt-Rh metals on catalyst surfaces, resulting in the formation of the remarkable catalytically active sites at the metal-support interface in the reduction of NH3 in this process. In addition, the nanosized Pt-Rh composite-induced cytotoxicity testing was mainly applied to the human lung MRC-5 cell line and the percentage of cell survival was determined by MTS analysis in vitro. For nanosized Pt-Rh composite, only minor cytotoxicity was observed when human lung cells were exposed.