Objective: To investigate possible causes of micromotion in distal femoral prosthesis. Methods: Based on the assumption that the femur and prosthesis were considered as concentric cylinders with completely bonded interface,a theoretical mode1 simulating the interfacia1 stress transfer was established.The distributions of the interfacia1 shear and radial stresses with the changing of z were obtained through mathematics and mechanics deducing. Results: The maximum interfacial shear stress occured at the position of z＝0,namely,the cross section of the femur neck.The interfacia1 shear stress sharply decreased with the increasing of z and came to nearly zero at the range of z＞0.1 m.While the interfacial radial stress increased with the increasing of z,at the range of z＞0.05 m it was constant and reached the maximum. Conclusion: The micromotion in distal prosthesis is caused by the interfacial radial stress.