Abstract: The relation is linear between axial supporting capacity of specimen and confining pressure,but the crack angle of specimen can not be given by Coulomb criterion.The axial supporting capacity of sections with angle around 45°+φ/2 is nearly the same based on the concept of cohesion and internal friction.So the real crack face usually resulted from the bedding and defect in the specimen.The failure of specimen for sandstone that is made of clearly particles is usually diagonal crack with part cone,but one for coal and limestone which is made of tiny particles is plane crack.For specimen with the ratio 2 of length to diameter,the plane with angle 45°+φ/2 will connect the upper and lower face of the specimen,not the surround face when the internal friction angle is larger than 36.87° or the influencing coefficient of confining pressure on the strength is larger than 4.And it is difficult to analyze the supporting character of specimen with Coulomb criterion.