A rotor assembly for a motor is disclosed. The motor has a minimum air gap width value. The rotor assembly includes magnets, a main-body portion, magnet-receiving slots and arc-trimming portions. Each of the magnet-receiving slots is disposed around a central axis of the main-body portion and locates through the main-body portion for accommodating the corresponding magnet therein. The magnet-receiving slot has a slot width value. The arc-trimming portions spatially correspond to the magnet-receiving slots. Each of the arc-trimming portions and a geometric symmetry center of the corresponding magnet-receiving slot together form an arc-trimming depth value. The arc-trimming depth value is greater than a sum value of a rate constant multiplied by the slot width value and then subtracted 2 times of the minimum air gap width value, and is less than the sum value of the rate constant multiplied by the slot width value and then plus 2 times of the minimum air gap width value.