Methods to determine EU(Ⅲ) ion in aqueous solution by fluorescence spectrometry based upon the ligand sensitized fluorescence of Eu(Ⅲ)-terephthalic acid (TPA) complex ion have been studied. The effects of excitation wavelength, pH, concentration of TPA and emission wavelength on the fluorescence intensity were investigated. The fluorescence intensity of the Eu(Ⅲ) complex ion was further increased with addition of trioctylphosphine oxide (TOPO). In this case Triton X-100 was used to dissolve TOPO in aqueous solution. The calibration curve for Eu(Ⅲ) was linear over the range from $1.0{ imes}10^{-6}M;to;4.0{ imes}10^{-4}M$ and the detection limit was $1.0{ imes}10^{-6}M$ under the experimental conditions of 256 nm, 5.6, $3.5{ imes}10^{-4}$M$ and 615 nm for excitation wavelength, pH, concentration of TPA and emission wavelength, respectively. When TOPO was added to the Eu(Ⅲ)-TPA system, the concentration range of linear response and the detection limit were $1.0 { imes}10^{-9}M;to;1.0{ imes}10^{-4}M,;1.0{ imes}10^{-7}M,$ respectively under the experimental conditions of 284 nm, 4.4 and $1.0{ imes} 10^{-4}M$ for excitation wavelength, pH and concentration of TOPO, respectively. Effects of interferences from various cations for the determination of Eu(Ⅲ) ion were also investigated.