The authors report the preparation of novel single-stranded DNA libraries with/without stable preorganized structures for the selection of structure-preorganized or structure-induced aptamers by the separation of random DNAs using capillary sieving electrophoresis. As explained in the manuscript, the obtained DNAs were found to be significantly different from each other, indicating their successful separation based on their stable preorganized structure under the described experimental conditions. Our efforts to establish a procedure for a distinct selection between structure-preorganized and structure-induced aptamers are currently under progress; further analysis is required to evaluate the selected aptamers. Nonetheless, the results obtained thus far yield an important technological advancement for the aptamer selection and will be of great help to not only analytical scientists but also researchers engaged in developing biosensors.Additionally, the first author will be applying to the 'Japan Society for the Promotion of Science Research Fellowship for Young Scientists' this spring. Therefore, we want to urgently report the developed method as a Rapid Communication in Analytical Sciences.