The structural response and melting of indium nanoparticles embedded in the ultrathin aluminum film were studied for slow thermal heating and for ultrafast laser heating using electron diffraction. The nanoparticles were prepared by magnetron sputtering. For slow heating, the indium nanoparticles lost their structural order ∼19 K below the bulk melting point of indium. Significant undercooling was observed when the nanoparticles were cooled. For ultrafast heating using a 110 fs laser pulse, a subset of the nanoparticles was superheated in the range of 1.15−1.40 T m when heated from 352 K. The temporal response of the indium nanoparticles to ultrafast laser excitation was found to be anisotropic.