Spectroscopic properties of Er3+ ions embedded in tellurite glass were investigated mainly as a function of MoO3 content. Emission intensity and full width at half maximum of the Er3+:4I(13/2) -> 4I(15/2) transition turned out to be maximized in tellurite glass with 10 mol% MoO3. Changes in the excited-state lifetime of Er3+:4I(13/2) level also showed a different behavior on the variation of MoO3 amount. Judd-Ofelt analysis was performed and the figure of merit for a gain medium was estimated. We used Er L-3-edge extended X-ray absorption fine structure analysis to elucidate the local environments of Er ions in tellurite glass with or without MoO3. The spectroscopic changes were discussed in connection with the structural parameters obtained.