The authors have already synthesized the noncircular planetary gear mechanisms with nonuniform motion by using the noncircular gears for the planetary gear mechanisms. In this paper, the dynamic characteristics of those mechanisms are made clear by analysis and experiment. First, the torque fluctuation occurring in the driving axle of the mechanism in operating the mechanism is analysed and its equation is induced. Next, the mechanism is modeled to a vibration system and the equation of motion is induced and the dynamic displacement curve is calculated. Moreover, considering the mechanism as an equivalent moment of inertia, the equation of motion is induced and the value of the torque fluctuation is calculated. And then, by comparing and discussing the theoretical results With experimental results, the basis of design of the mechanisms is made clear.