In this paper, platinum(Pt)-selenium(Se) nanostructures and Pt nanoparticles were synthesized by a facile one-step chemical reduction route and their catalytic performance was evaluated as oxidase mimic. The results of structure characterization revealed that Pt-Se nanostructures consist of Pt and Se atoms (the Pt/Se atomic molar ratio is approximately 7: 3), while Pt nanoparticles consist of pure element Pt. The oxidase-like activity of Pt-Se nanostructures and Pt nanoparticles was evaluated with 3,3',5,5'-tetramethylbenzidine (TMB) as substrate. The results indicated that Pt-Se nanostructures had a lower Michaelis constant (K-m) and higher catalytic constant (K-cat) for TMB oxidation than that of Pt nanoparticles, which mean the binary Pt-Se hybrid nanostructures had stronger binding affinity with TMB and higher catalytic activity in comparison with monometallic Pt nanoparticles. The enhanced oxidase-like activity of Pt-Se nanostructures may be due to element Se doped in binary Pt-Se hybrid nanostructures, which can accelerate electron transport and provide excellent chemical stability against catalytic performance degradation during the TMB oxidation reaction. Mercury (II) ions(Hg (2+)) could inhibit the oxidase mimetic activity of Pt-Se nanostructures and resulted in a color change of the reaction system. Based on this mechanism, a facile colorimetric assay for Hg2+ was developed with a detection limit as low as 70 nm and a linear range of 0-2.5 mu M.