The Phase stabilities of nanometer-sized materials are quite different from those of the corresponding bulk materials. Among the phase stabilities, melting point suppression is one of the most fundamentally important issues. Until now, the melting point suppression was studied with nanoparticles supported by solid substrates. However, the substrate effect on melting point of the nanoparticles has not been taken into consideration. In this work, real-time, atomic-scale direct observation of melting point suppression in nanometer-sized gold and silver particles on graphite substrate, along with simple size reduction, was conducted by in-situ high resolution electron microscopy. Namely, it was confirmed that a crystalline-to-liquid transition occurred when the size of gold particle, being placed on graphite substrate maintained at 1100 K, decreased down to 5 nm during diminution. However, no crystalline-to-liquid transition, only a crystalline-to-gas transition, took place in graphite-supported silver nanoparticles prior to the complete disappearance of the particles.