A new mechanism construction for small tooth number difference planetary gear drive is developed in which the planet wheel is guided by a planar crank and oscillating block mechanism. The sizes of linkage are design dexterously to get an approximate circumference linkage curve so that the engaging condition of internal gear pair can be satisfied. The trajectory of the inner gear center motion is analyzed and its error comparing with a standard circle is calculated to avoid movement interference. The movement of inner gear is study particularly to deduced formula of instantaneous transmission ratio. Despite observable fluctuation of output speed, this new type of gear transmission mechanism still has potential application value in situation with large ratio and low input speed. A hand drive winch prototype using the mechanism is also illustrated in this paper.