A series of Eu3+-doped polyimide nanocomposites were successfully prepared via the solution process. X-ray diffraction, differential scanning calorimetric analysis, thermogravimetric analysis, transmission electron microscopic analysis, universal testing, and ellipsometry were used to characterize the nanocomposites. TEM results indicated that rare earth particles were homogeneously dispersed in the polyimide matrix with a size of about 20 nm. The introduction of the rare earth complex led to an increase in the glass transition temperature, thermal stability, size stability, and a marked increase in the tensile strength and storage modulus. The addition of the rare earth complex also increased the refractive index of the nanocomposites.