We previously proposed a passive mechanism as the link knee joint unit (LKJ) for a transfemoral prosthesis for stair ascent. The prototype allowed the experimental subjects to ascend stairs without the use of a handrail. In the present study, we modified the LKJ unit and developed further two designs of the LKJ unit. One has full knee extension function during the prosthetic stance phase (condition 1). The other design mechanically trades off the functional range of knee extension against stability of the LKJ unit (condition 3). In the stair ascent experiment with six able-bodied subjects, all subjects succeeded in ascending stairs with the three LKJ conditions without the use of a handrail. No difference was found in joint angels and joint moments of the intact and prosthetic legs among all LKJ conditions. However, subjective assessment for ease of LKJ extension during stair ascent showed that the participants felt easier to extend the LKJ unit in the condition 1 and 2 than the condition 3. It is suggested that the condition 1 or 2 is appropriate for prosthesis users who can ascend stairs with the LKJ unit. For prosthesis users who are not familiar with the LKJ unit, the condition 3 would be useful to learn how to use it.