Abstract: PVA/[Y(NO_3)_3+Er(NO_3)_3] composite nanofibers were fabricated by electrospinning.Y_2O_3∶Er~(3+) upconversion nanofibers were obtained by calcination of the relevant composite nanofibers. XRD analysis revealed that PVA/[Y(NO_3)_3+Er(NO_3)_3] composite nanofibers were amorphous in structure, and Y_2O_3∶Er~(3+) upconversion nanofibers were cubic in structure with space group Ia3. SEM images indicated that the mean diameter of the composite nanofibers was 130 nm, and Y_2O_3∶Er~(3+) upconversion nanofibers of 60 nm in average diameter were acquired at 600 ℃. TG-DTA analysis revealed that the water,organic compounds, nitrates in the composite nanofibers were decomposed and volatilized totally, and the weight of the sample kept constant when sintering temperature was above 600 ℃, and the total weight loss percentage was 80;. FT-IR analysis manifested that the spectrum of the composite nanofibers was basically the same as that of the pure PVA, and Y_2O_3∶Er~(3+) upconversion nanofibers were formed at 600 ℃.The upconversion spectroscopic properties of the Y_2O_3∶Er~(3+) nanofibres were investigated under the excitation of a 980 nm continuous wave diode laser. The Y_2O_3∶Er~(3+) nanofibers emitted strong green and red upconversion emissions centering at 522 nm,561 nm and 658 nm, respectively. The green emissions were attributed to the transitions of ~2H_(11/2)/~4S_(3/2)→~4I_(l5/2) energy levels of Er~(3+) , and the red emission was assigned to the transition of ~4F_(9/2)→~4I_(l5/2) energy levels of Er~(3+) . The formation mechanism of Y_2O_3∶Er~(3+) upconversion nanofibers was advanced. The technique could be applied to fabrication of other rare earth composite oxides upconversion nanofibers.