Abstract: Corner rounding modification is commonly used to improve the aerodynamics of a square cylinder. However, its flow mechanism has not been clarified yet. Aerodynamic performances and flow field characteristics of sharp and rounded-corner square cylinders have been investigated using large eddy simulation (LES) in a uniform flow at the Reynolds number of 2.2\times 10^4. The effect of attack angle has been evaluated, and characteristics of the shear layer and the separation bubble have been quantitatively analyzed. The physical mechanism behind the corner modification is discussed as well. Results show that the overall surface pressures, aerodynamic forces and vortex shedding intensity of rounded-corner square cylinders have a downward trend in comparison to sharp-corner cylinders. The "separation bubble'' flow pattern occurs in a lower attack angle for rounded-corner cylinders. As the increase of the attack angle, the length of the separation bubble decreases until it disappears. Besides, the separation point of rounded-corner cylinders moves downstream, which leads to a thinner shear layer and a narrower wake and a weaker vortex shedding. All these factors result in a reduced drag coefficient and an increased Strouhal number of rounded-corner square cylinders.