Slag products derived from the pyrolysis/melting and plasma/melting treatment of municipal solid waste (MSW) in Japan were examined for the characterization study of heavy metal-bearing phases using petrographic techniques. Detailed microscopic observations revealed that the shapes of heavy metal-rich inclusions are generally spherical to semi-spherical and their sizes range from submicron to scarcely large size spheres (over 100 microm). The experiments (both optical microscopy and electron probe microanalysis) indicated that Fe and Cu participate in mutual substitution and different proportions, and form mainly two-phase Fe-Cu alloys that bound in the silicate glass. This alloy characterizes the composition of more than 80% of the metal-rich inclusions. Other metals and non-metals (such as Pb, Ni, Sb, Sn, P, Si, Al and S) with variable amounts and uneven distributions are also incorporated in the Fe-Cu alloy. In average, the bulk concentration of heavy metals in samples from pyrolysis/melting type is almost six times greater than samples treated under plasma/arc processing. The observations also confirmed that slag from pyrolysis origin contains remarkably higher concentration of metallic inclusions than slag from plasma treatment. In the latter, the metallic compounds are separately tapped from molten slag during the melting treatment that might lead to the generation of safer slag product for end users from environmental viewpoint.