The significance of supported metal catalysts in heterogeneous catalysis calls for in-depth understanding of metal particle sintering behavior in order to rationalize the design of stable nanocatalysts. Here, we show that the sintering behavior of supported metal nanoparticles (NPs), exemplified by studying the carbon-supported Pt NPs, becomes markedly accelerated when the interparticle distance between neighboring metal NPs is shortened. Pt NPs with close proximity are prone to form aggregates and further coalescence to fewer, larger crystals under high temperature conditions. Enlarging interparticle distance on the support and engineering NP surface with overcoatings are beneficial for the preparation of sinter-resistant catalysts. The former can be achieved through reducing the metal loadings or utilizing suitable synthetic technologies that allow for the homogeneous distribution of supported metal particles. The surface overcoating engineering relied on the carbon nanoshell formed on the Pt particle surface by in situ pyrolysis of an organic polymer. The evolution of carbon nanoshell formation was visualized by in situ transmission electron microscopy measurements. We also show the potentials of surface overcoating for enhancing catalytic performance.