Abstract: The cone/bit speed ratio of tri-cone bits play an important role in conduction simulation research, directing drill bits design and selection. Theoretically, the correlation of the geometrical structure of and the load on teeth of the bits is analyzed. It is determined that there is a driving tooth row on the roller cone. A computer simulation model is established based on the minimum power theory. Subsequently, the model is modified because of the influence of bit load and rock properties on ratio. Finally, the experiment of drilling is conducted to measure the actual value of the cone/bit speed ratio when the rock bits break sand rock, and the computer simulation model is used to compute the cone/bit speed ratio of the same bits. The simulation results are compared with the experimental data. Results show that the simulation model can be used to compute the cone/bit speed ratio accurately.