Pt-Pd alloy nanoparticles exhibit better catalytic activity and selectivity than pure Pt and Pd ones, and thus to explore their stable structures is crucial for understanding the catalytic performance of nanoparticles. In this paper, the particle swarm optimization algorithm and the quantum Sutton-Chen potentials are employed to investigate the stable structures of tetrahexahedral Pt-Pd alloy nanoparticles with different sizes and different composition. Results show that in Pt-Pd alloy nanoparticles, Pt atoms are preferential to locate of the core and Pd atoms to occupy the surface. Furthermore, the more symmetrical and ordered the structure, the lower the energy of the structure. The Warren-Cowley chemical short-range order parameters of three different sizes of nanoparticles increase accordingly with rising fraction Pt. The segregation degree of small sized nanoparticle is more remarkable than large sized one for the same content of Pt.