The effects of aspect ratio and Reynolds number on aerodynamic characteristics of three-dimensional rectangular wing at low Reynolds number of 103 to 105, are investigated with Reynolds-averaged Navier-Stokes solver with the Baldwin-Lomax model. Present results show that lift coefficient decreases drastically at lower aspect ratio than 4. Besides, the much larger viscous drag coefficient is obtained at the lower Reynolds number, especially lower than 104. In order to focus on designing practical wings, the particular cases under the condition of fixed wing-surface area and fixed main stream velocity are conducted. The results show that there is trade-off between the decrease in viscous drag coefficient with increasing Reynolds number and the increase in lift coefficient with increasing aspect ratio. At the lower Reynolds number condition, as the former effect is stronger than the latter one, maximum lift-to-drag ratio is obtained at lower aspect ratio.