Optical whispering gallery modes were observed in micron-sized SrS:Eu(2+) octahedrons. Cathodoluminescence in a scanning electron microscope was used to evaluate the emission characteristics of individual octahedrons with various sizes. The resonance modes superposed on the orange Eu(2+) broadband emission were explained by a plane wave model, taking the wavelength dispersion of the refractive index into account. The resonance modes occur in the three equatorial planes in the octahedrons, which were approximated by corner-cut square disks.