Abstract: How to determine the friction force of a rolling rigid-body often confused the students who are learning engineering mechanics. The reason that results in the friction force was analyzed, and thenthe equation to determine the static friction force was ascertained. By means of the work-energy principle, the effect of friction force was discovered. It was concluded that the total work of the static friction force of a rolling rigid-body is zero and the total work of the sliding friction force of a rolling rigid-body is negative. The role of the friction force is to realize the conversion between the rotation kinetic and the stable motion kinetic energy. The direction of the static friction force is to prevent the trend of motion due to force, and the direction of the sliding friction force is negative to the direction of sliding movement.