Abstract: A harmonic drive includes a housing, a first gear, a second gear, and a driving unit. The first gear has a first number of teeth. The second gear is fixed to the housing, and has a second number of teeth. The second number is different from the first number. The driving unit is configured to drive the first gear to rotate when rotating. The driving unit includes a supporting base, and a plurality of third gears affixed to the supporting base in a rotatable manner, and each of the third gears is engaged with the first gear and the second gear.