A harmonic drive includes a circular spline, a cup-shaped flex spline meshed with the circular spline, a wave generator including an bearing mounted in the cup-shaped flex spline, an elliptical wheel mounted in the bearing and a coupling connected to the elliptical wheel so that an oil storage chamber is defined between the wave generator and the cup-shaped flex spline, and one or multiple stirrers connected to the coupling of the wave generator and drivable by the coupling to carry the storage lubricating oil out of the oil storage chamber for lubricating the bearing of the wave generator and the meshed tooth surfaces between the circular spline and the cup-shaped flex spline.