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Uniform spheres are highly desired for the fabrication of advanced ceramics and as phosphor for lighting and display. Green emitting (Y 0.95− x Gd x Tb 0.05 ) 2 O 3 phosphor spheres were successfully synthesized in this work via homogeneous precipitation, with particular emphases on size enlargement of the particles and the effects of particle size, Gd 3+ incorporation and calcination on photoluminescence. Ammonium nitrate (NH 4 NO 3 ) was originally used to effectively enlarge the precursor particles from ∼150 ± 10 to 590 ± 43 nm, whose effects were found to tend to saturate at NH 4 NO 3 :rare-earth = 1:1 molar ratio. Size enlargement up to ∼630 ± 57 nm has also been achieved via a self-seeded growth approach. The optimal Gd 3+ content for luminescence was determined to be 2 at% ( x = 0.02) for the phosphors, and the spin-allowed 7 F 6 → 7 D J Tb 3+ excitation at ∼278 nm successively lowers at a higher Gd 3+ content or calcination temperature owing to bandgap narrowing of the host lattice. Intensity of the 543 nm green emission steadily improves with increasing temperature of calcination or particle size, and the lifetime of the 543 nm emission, affected by particle size and calcination, falls in the range of ∼1.75–2.81 ms. The phosphors have similar color coordinates of around (0.32, 0.56), irrespective of the Gd content, particle size, and calcination temperature. [ABSTRACT FROM AUTHOR]