A manipulator arm mechanism includes a pedestal, a driving member, a speed reducer, and an output rocker arm. The driving member is mounted on the pedestal and has an output shaft. The speed reducer is assembled to the pedestal and connects with the output shaft of the driving member. The speed reducer includes a first transmission assembly, a second transmission assembly, and a third transmission assembly engaging each other in turn. The third transmission assembly is a planetary gear set and includes a sun gear rotated by the second transmission assembly, a ring gear, and a plurality of planetary gears. The ring gear is coaxially sleeved on the sun gear. The planetary gears are assembled between and rotatably mesh with the sun gear and the ring gear. The output rocker arm is fixed with the plurality of planetary gears.