Abstract: The direction of the friction force is determined when a homogeneous circle rolls without slipping on horizontal plane under the action of the couple or the horizontal force. Only by the couple made pure rolling circle, fraction is the only force causing the linear acceleration and has a moment about the mass center whose direction is reversed to of the couple. Then, the fraction force also decreasing the angular acceleration. When the circle is acted only upon by a horizontal force, using rigid planar differential equations of motion and theorem of kinetic energy, the direction of fraction force is determined by the acting location, the radius of the cylinder and radius of gyration.