The roll motion of the vehicle has an effect on the yaw natural frequency through its change of slip angle. Because the yaw natural frequency formula considering this effect is the solution to a quartic equation, this paper created approximate solution formulas for yaw natural frequency and its damping ratio which take roll motion into account. The method is as follows. The change in the slip angle was converted into the lateral stiffness of tires. This stiffness was converted into the relaxation time of the tire. This time was added to the natural frequency and the damping ratio by Complex Cornering Stiffness. Further this paper considered the approximation accuracy and valid range of these formulas. Finally this paper pointed out that roll motion increases yaw natural frequency, and at vehicle speeds higher than a certain speed it also decreases the fast response.