A grass trimmer has a main body and a trimming device. The main body has an operating device and a power source. The trimming device has a grass cutting assembly having a cutting member and a power head for rotating the cutting member. The power head has a housing, a rotor assembly and a stator assembly. The housing is formed integrally and defines an accommodation chamber. The rotor assembly is received in the accommodation chamber of the housing and has a rotor sleeve which rotates around an axis. The stator assembly is at least partly received in the rotor sleeve. The rotor assembly further has two or more magnetic pieces disposed at an inner wall of the rotor sleeve. The stator assembly has a stator support and a coil assembly. The stator support includes a mounting portion and a supporting portion. The coil assembly is disposed in the rotor sleeve by the supporting portion.
