Abstract A new technique to obtain the oscillator strength of select rare-earth optical transitions in nanostructured dielectric materials (nanophosphors) is presented. It is based on the experimentally observed nanophosphor lifetime dependence on the embedding medium. A constant oscillator strength and parity-allowed electric dipole transitions of the RE ion emission are assumed. The oscillator strength is obtained from the slope of the 1 / τ i j vs. n ( n 2 + 2 ) 2 plot, where τ i j is the radiative lifetime of transition between states i and j , and n is the index of refraction of the embedding medium. The use of the technique is illustrated for the Y 2SiO5:Ce nanophosphor.