1. A yaw angle of tractor with no steering angle was calculated from the side slip angle of pneumatic tire, keeping the direction to the contour line of slope field.
2. The relation between side slip angle of driving tire and transformed side force was expressed by means of an exponential curve, where transformed side force meant side force divided by the coefficient of resistance force for side slipping.
3. But in the larger range over the adhesion of rolling tire on the running surface, the tractor could not develope the traction force, because of larger sideway slipping of tire. In the range under the adhesion, the relation stated above was the same to the one of the rolling tire, and was in linear.
4. From this characteristics, the yaw angle, with the same side slip angle of front tire to rear tire, was in larger value with increase of the traction force and inclination angle of the slope jointly, but side slip angle was nearly in the same value for all traction force.
5. For the purpose of running along the contour line, the yaw angle is needed to be equal to the side slip angle. But it does not always exist, but occasionally, only at a certain traction force at each inclination angle respectively.