Abstract: A kind of miniature roller reducer with small volume, light weight and simple structure was studied. The kinematic design of miniature roller reducers was approached based on the tribological principle and the theory of the equivalent mechanisms. The equivalent mechanisms were supplied and the transmission ratio equations were given. The results showed that the transmission ratio is relative to the number and the size of the rollers. The structure type with three row of rollers and three contacting points is advantageous to change the transmission ratios and to increase the load capacities.