Thermal annealing is an important and widely adopted step during the synthesis of Pt bimetallic fuel-cell catalysts, although it faces the inevitable drawback of particle sintering. Understanding this sintering mechanism is important for the future development of highly active and robust fuel-cell catalysts. Herein, we studied the particle sintering during the thermal annealing of carbon-supported Pt1–xNix (PtNi, PtNi3, and PtNi5) nanoparticles, a reported recently class of highly active fuel-cell catalysts. By using high-resolution transmission electron microscopy, we found that annealing at an intermediate temperature (400 °C) effectively increased the extent of alloying without particle sintering; however, high-temperature annealing (800 °C) caused severe particle sintering, which, unexpectedly, was strongly dependent on the composition of the alloy, thus showing that a higher Ni content resulted in a higher extent of particle sintering. This result can be ascribed to the solid-state transformation of the carbon support into graphene layers, catalyzed by Ni-richer catalyst, which, in turn, promoted particle migration/coalescence and, hence, more-significant sintering. Therefore, our results provide important insight for the synthesis of carbon-supported Pt-alloy fuel-cell catalysts.