Abstract: The nature of formation is an important factor to affect the bit trajectory, so that the hole deviation problems are taken place both in straight and directional drilling. To control the bit trajectory satisfactorily, it is necessary to determine not only the “assembly force” by selecting an appropriate bottom hole assembly (BHA) lying in a definite shape of borehole, but also the "formation force" due to the interaction between the bit and the formation. The drillers had long been embarrassed by the difficulty of keeping the hole straight in vertical drilling. Herein, try to calculate the formation forces to change the angles of inclination and azimuth quantitatively at the Unity Oil Field-Sudan.