Herein, La2Zr2O7:5% Eu3+ nanoparticles (NPs) with different sizes have been synthesized for the first time through a modified facile molten salt process using a single-source complex precursor of La(OH)(3)ZrO(OH)(2):Eu(OH)(3)nH(2)O. It was found that the concentration of the added ammonia to co-precipitate the corresponding metallic ions to form the precursor can influence the final particle size of the fluorite La2Zr2O7:5%Eu3+ NPs. Furthermore, the crystal phase of the La2Zr2O7:5%Eu3+ NPs was transferred from fluorite to pyrochlore after thermal treatment at 1000 degrees C. The relationship between photoluminescence (PL), quantum yield (QY), particles size and crystal phasehas been further investigated through fluorescence decay, site symmetry, and Judd-Ofelt (J-O) analysis. Specifically, PLQY and lifetime increase with increasing particle size of the fluorite La2Zr2O7:5%Eu3+ NPs. Additionally, crystal phase transfer from fluorite to pyrochlore resulted in large PLQY decrease and moderate lifetime increase in the La2Zr2O7:5%Eu3+ NPs.