A power tool has a casing and a motor assembly installed in the casing. The motor assembly comprises a motor and a heat dissipation device. The motor comprises a housing and magnets or windings disposed at an inner surface of the housing. The heat dissipation device is arranged at an outer surface of the housing and a window is formed in the casing to expose the heat dissipation device. Preferably, the heat dissipation device comprises an annular heat absorbing section fitted to a radially outer surface of the motor housing and a plurality of fins extending from the heat absorbing section and into the window.