Crystal structure deviations of bimetallic Pd-Pt nanoparticles (NPs) were investigated using synchrotron high-energy X-ray diffraction. The samples comprised Pd NPs and bimetallic Pd-Pt NPs featuring those with a Pd-core/Pt-shell structure as well as PdPt solid-solution NPs with varying diameters of 6.1, 6.7, 8.1, and 11.2 nm. The Rietveld method was used to find the lattice constant of the Pd NPs, which was larger than the bulk value; while reverse Monte Carlo modeling revealed they possessed a highly disordered structure. The lattice constants of the Pd-Pt bimetallic NPs were found to vary with the NP Pt content. For Pd-core=Pt-shell structure NPs, the lattice constants decreased with Pt content owing to lattice relaxation at the core/shell interface.