In this study, the formation characteristics of aerobic granular sludge (AGS) mainly composed of nitrifying bacteria (referred to as nitrifying granules) in a continuous stirred-tank reactor (CSTR) were investigated at several surface loading rates (equal to the liquid linear velocity at the sludge settling zone). The surface loading rate strongly affected the selection of larger granular sludge similarly to the sludge settling time in a sequencing batch reactor. By setting an appropriate surface loading rate (1.4 m³/m²/d), small particles were effectively washed out, and larger granular sludge selectively remained in the reactor. As a result, nitrifying granules were effectively formed even with keeping an identical flow condition from the startup. To the best of our knowledge, this is the first study that has succeeded in forming nitrifying granules with keeping an identical flow condition from the startup using a continuous-flow reactor. These findings will contribute to the dissemination of AGS technology because information on the formation of AGS in a continuous-flow reactor is limited.