Abstract: Aqueous sodium-ion battery is a complementary technique to lithium-ion battery because of its comparatively low cost, improved safety, and environmentally friendly electrolyte. However, the lower electrode capacity limits the application of this kind of batteries. Na_0.44MnO₂ is a high capacity cathode for Na-ion batteries. Its theoretical capacity is 121 mAh/g. In this article, we investigate the size effect of Na_0.44MnO₂ on cathode performance. The nanorods are prepared by heat treatment of a MnO₂ nanoflake precursor, of which the size is tuned by the ratio of CTAB and KMnO₄. The Na_0.44MnO₂ nanorods are then used as the active material of cathode for aqueous sodium-ion batteries. The nanorod cathode delivers 60 mAh/g in the initial cycle at 1 C and retains 55 mAh/g after 200 cycles, which is 37.5% higher than Na_0.44MnO₂ bulk cathode. This cathode gives a high capacity of 47 mAh/g after 200 cycles at a high rate of 5 C. The increased capacity is attributed to diminishing charge transfer resistance and improved Na-ion diffusivity caused by the higher specific surface area of the nanorod.