Two series of solid complexes of europium and terbium with biphenyl 4-carboxylic acid and phen were synthesized and characterized in this report. Their elemental analysis, molar conductivities and TG-DTA studies indicate that the complexes have the composition of Eu(phen)L3·1/2H2O, Eu0.5RE0.5(phen) L3·1/2H2O; Tb (phen) L3·H2O and Tb0.5 RE0.5(Phen)L3·1/2H2O. (RE = Y3+, La3+ and Gd3+; L = biphenyl 4-carboxylic acid; phen = o-Phenanthroline). The studies of their IR, UV 1H NMR and molar conductivities demonstrate that biphenyl 4-carboylic acid is bounded with RE (III ) ion. Rare earth ions coordinate with two nitrogen atoms of phen molecules directly in these rare earth complexes. The fluorescence spectra and fluorescence lifetimes of the rare earth complexes show that the fluorescent intensity and lifetime of a series of europium complexes are longer than those of the series of terbium complexes as having the some ligands. There are better fluorescent intensity and lifetime of hetero-nuclear rare earth complexes than homo-nuclear rare earth complexes for europium complexes. The fluorescence emission intensity of Eu3 + is raised by inert fluorescent rare earth ions (Y3+ , Gd3+ and La3+ ), but in Tb3+ hetero-nuclear rare earth complexes the intensity of Tb3+ ions are quenched by the inert fluorescent rare earth ions. Two series of solid complexes of europium and terbium with biphenyl 4-carboxylic acid and phen were synthesized and characterized in this report. Their elemental analysis, molar conductivities and TG-DTA studies indicate that the complexes have the composition of Eu (phen) L3. (RE = Y3 +, La3 +, and Gd3 +); Tb (phen) L3 · H2O and Tb0.5 RE0.5 (Phen) L3 · 1/2H2O; Eu0.5RE0.5 The studies of their IR, UV 1H NMR and molar conductivities demonstrate that biphenyl 4-carboylic acid is bounded with RE (III) ion. Rare earths ion coordinate with two nitrogen atoms of phen molecules directly in these rare earth complexes. The fluorescence spectra and fluorescence lifetimes of the rare earth complexes show that the fluorescent intensity and lifetime of a series of europium complexes are longer than those of the series of terbium complexes as having the some ligands There are better fluorescent intensity and lifetime of hetero -nuclear rare earth complexes for homo-nuclear rare earth complexes for europium complexes. The fluorescence emission intensity of Eu3 + is raised by inert fluorescent rare earth ions (Y3 +, Gd3 + and La3 +), but in Tb3 + hetero-nuclear rare earth complexes the intensity of Tb3 + ions are quenched by the inert fluorescent rare earth ions.