Metallic sulfide fullerene Sc 2 S@C 90 has been detected by mass spectra but not been isolated and structurally characterized. It is impossible to perform extensive tests on Sc 2 S@C 90 due to its low yield, quantum chemical calculations thus become an important method to predict or identify its structure and properties. Here a systematic density functional theory study are first performed on 15756 isomers of fullerene C 90 and their anions with 0～3 pairs of fused-pentagons, then 30 isomers of metallic sulfide fullerenes Sc 2 S@C 90 are constructed by putting Sc 2 S into the selected candidate cages with different orientations and geometrical optimizations are performed on these isomers with the density functional theory method. The calculated results demonstrate that the two lowest-energy isomers are Sc 2 S@C 90 :99913 and Sc 2 S@C 90 :99915, respectively. Both isomers satisfy the isolated-pentagon rule. To clarify the relative stabilities of the five lowest-energy isomers of Sc 2 S@C 90 at high temperatures, enthalpy-entropy interplay has been taken into consideration with respect to the temperature range of up to 4000 K; the calculations demonstrate that the two lowest-energy isomers of Sc 2 S@C 90 may coexist in the soot at high temperatures. Structural analysis demonstrated that the two isomers can transfer into each other by two Stone-Wales rotations, further suggesting the possibility of interconversion between them. Molecular orbital analysis indicates that Sc 2 S cluster transfers four electrons to the cage; however nature charge analysis demonstrates the charges transferred from the encaged cluster to the parent cage is much smaller than that with the simple molecular orbital analysis. The quantum theory of atoms in molecules is used to investigate the connectivity and interaction nature between the encaged cluster and parent cage. The results show that there are strong interactions between the encaged cluster and parent cage. The simulated infrared spectra of the two lowest-energy isomers are provided to assist future experimental identification and characterization of the structure of Sc 2 S@C 90 .