The objectives of the work reported in this article were to develop a novel 6-degree-of-freedom (DOC) robotic system for knee joint biomechanics, to complete a hybrid force-position control scheme, to evaluate the system performance, and to demonstrate a combined loading test. The manipulator of the system utilizes two mechanisms; the upper mechanism has two translational axes and three rotational axes while the lower mechanism has only a single translational axis. All axes were driven with AC servo-motors. This unique configuration results in a simple kinematic description of manipulator motion. Jacobian transformation was used to calculate both the displacement and force/moment, which allowed for a hybrid control of the displacement of, and force/moment applied to, the human knee joint. The control and data acquisition were performed on a personal computer in the C-language programming environment with a multi-tasking operating system. Preliminary tests revealed that the clamp-to-clamp compliance of the system was smaller in the vertical (Z) and longitudinal (Y) directions (0.001 mm/N) than in lateral (X) direction (0.003 mm/N). The displacement error under the application of 500 N of load was smallest in the vertical direction (0.001 +/- 0.003 mm (mean +/- SD), and largest in the lateral direction (0.084 +/- 0.027 mm). Using this test system, it was possible to simulate multiple loading conditions in a human knee joint in which a cyclic anterior force was applied together with a coupled, joint compressive force, while allowing natural knee motion. The developed system seems to be a useful tool for studies of knee joint biomechanics.