The cone penetration test(CPT) can’t be used in harder soil such as loess or in deeper soil because of the higher cone resistance and the limited capacity of the equipment of CPT. Cone’s being smoothly pushed into the ground is prerequisite for applying probing technologies in harder or deeper soil. By theoretically analyzing the forces on the cone

the theoretical equations of total cone resistance and torque

excluding wall sided frictional resistance

is established

in which all the influence factors are given. Meantime the mathematic equation to determine the friction coefficient between the soil and the cone is deduced from the theoretical equations of total cone resistance and torque. In addition

the laboratory rotary cone penetration test (RCPT) is carried out. The validity of theoretical analysis is proved. According to the results of RCPT for the samples with different water contents and different dry densities

linear relationship exists between the toque and the total cone resistance on the cone. Although the calculated value of friction coefficient between the soil and the cone based on the results of RCPT is much greater than that of internal friction coefficient of soil

its relevance to the internal friction coefficient of soil appears in some extend. The reason why the lower total resistance acts on the cone when the cone is simultaneously pushed and rotated than that when being only pushed is given by taking account of property along the vertical different from that in the horizon plane of soil and existence of frictional resistance between the cone and soil.