Thorium-uranium dioxide nanofibers were prepared in a three-step process. Green composite microfibers were electrospun from solutions composed of Th(IV) and U(VI) acetylacetonate complexes in different molar ratios, polyvinylpyrrolidone, and acetone/ethanol mixed solvents. The second step converted the green composite fibers by calcination in the air to mixed Th/U oxide fibers. In the final step, the heat treatment under forming gas provided the nanofibers with diameters of 28−46 nm composed of solid solutions ThxU1−xO2 (x = 1, 0.75, 0.5, 0.25, 0). The XRD and STEM-EDS analyses of the prepared Th/U mixed oxide nanofibers attested to their atomic homogeneity.