Electrokinetics provide an effective molecular manipulation technique in micro/nano fluidic environments, which match the length scale of various biological objects. In this work, two circular gold electrodes are fabricated on a glass substrate as DNA concentrators. A combination of alternating current (ac) electro-osmosis and electrophoresis is used to concentrate single-stranded DNA (ssDNA) molecules as small as 20 nucleotides in length. To understand the phenomenon of ac electro-osmotic flow, numerical simulation of the flow field is done and shows that the effective fluid flow is up to above the electrode surface. Hence, ssDNA molecules labeled with fluorescent dye are utilized to demonstrate the concentration effect on the DNA concentrator. ac electro-osmotic flow induced by the ac electric field can stir the bulk fluid, and ssDNA molecules can be transported from a large effective region to the surface of DNA concentrator. In addition, electrophoretic force induced by direct current (dc) bias is applied simultaneously to attract and hold ssDNA molecules. The motion of ssDNA molecules under electrokinetic forces is observed under a fluorescence microscope. The experimental results show that the ssDNA molecules can be concentrated on the electrode surface instantly. ssDNA concentration under different conditions is also compared and the results generated are discussed.