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The local environment of Mn4+ ions in CaAl12O19 has been studied by electron spin resonance (ESR) measurements and by spin-polarized first principles calculations within a density functional theory to determine the site that is occupied by the Mn4+ ion when substituted for the Al3+ ion in CaAl12O19. There are three crystallographically inequivalent octahedral sites available for the Mn4+ ion in the CaAl12O19 lattice. Crystal field calculations of the Mn4+ energy levels for the preferred site were performed and shown to be in good agreement with the experimental spectroscopic data. ESR measurements confirmed the tetravalent oxidation state of Mn ion and showed that the local environment of the Mn ion was independent of the Mn4+ concentration in CaAl12O19. The ESR measurement also suggested the multiple site occupation by the Mn4+ ion in CaAl12O19. Additionally, the optimum concentration of Mn4+ in CaAl12O19 that produced the brightest phosphor was determined.