Abstract: Stability of pillars is the basis of safety and efficient mining in underground mines,and it is the safety guar- antee for operators. Based on the theory of elastic mechanics,the stress state of inclined pillars was analyzed under the combined action of normal stress and shear stress,and a stress circle was plotted by graphic method which describes the distribution of stress state of pillars. According to the relationship between stress circle and Mohr-Coulomb strength envelope,an analytical formula for the safety factor of pillars was established. Some factors had been taken into consid- eration in the formula,such as the dip angle of ore body,the average normal stress and shear stress of pillars etc. The analytic formula of safety factor can be used to evaluate the stability of the pillars under any angle ore body. In the flat seam case,the stability of the pillars was evaluated by the safety factor. Results show that it is in good agreement with the measured values. Finally,the pillars stability influence factors were studied,including the deposit dip angle θ with the slope of the stress circle (tanβ). Research shows that tanβ is a binary function of the deposit dip angle θ and the lateral pressure coefficient λ. Tanβ can achieve only one maximum value under any lateral pressure coefficient value, but lateral pressure coefficient is different,getting the most value is also different. When the lateral pressure coefficient is from 0 to 1,the corresponding range of θ is from 45° to 90°. When the maximum value of tanβ is obtained,the safety factor is the smallest,and the pillar is most likely to be unstable. With the dip angle of ore body increasing,the safety factor decreases at first and then increases.