The efficiency analytical model of the inverted planetary roller screw mechanism is formulated based on the friction torque calculation. Firstly, the friction torque model considering the load distribution of thread teeth is established including the components such as curvature friction torque, friction torque due to spinning sliding, pure sliding friction torque between rollers and carriers, friction torque generated by the viscosity of lubricant, and preload torque. Secondly, the contribution of the friction torque components and torque distribution on the roller thread teeth are investigated. Finally, efficiency analysis is conducted by discussing the influences of structure parameters of the inverted planetary roller screw mechanism and operating conditions. Computational results reveal that the total friction torque and efficiency are influenced by axial load, number of roller thread teeth, nut speed, contact angle, and helix angle of nut with different extents. This study provides an understanding of the relationship between friction torque, transmission efficiency, and system design parameters, which can be employed to enhance the inverted planetary roller screw mechanism optimization design.