The emission spectra, excitation spectra and fluorescence decay curves of Eu3+:ThO2 are reported and analyzed. The Eu3+:ThO2 crystal was grown by the flux technique. Using selective dye laser exci tation at 12K and selection rules, site symmetries of Eu3+ ions in T hO2 are determined as Oh and C3v. Their crystal field levels and lifetimes were measured and tabulated. The dependence of energ y transfer and energy level position on temperature are also discussed.