The phenomenon of fluorescence enhancement for the system of terbium(Ⅲ) with terephthalic acid (benzene-1,4-dicarboxylic acid (TPA)) in the presence of an excess of La3+ in Triton X-100 aqueous solution has been studied. Thefluorescence intensity of Tb[1]-TPA system can be greatly increased by La3+. The intermolecular transfer of energy was re-sponsible for the mechanism of the fluorescence enhancement. Triton X-100 plays an important role for the stabilization ofthe system. The system, having the maximum excitation and emission wavelengths at 256 nm and 546 nm, respectively,shows a constant fluorescence intensity over the pH range between 4-6. The fluorescence intensity is a linear function ofTb3+ concentration in the range of 1.00 × 10-8-2.56 × 10-5 mol@L-1 and the limit of detection is 5.00 × 10-9 mol. L-1.