This paper concerns the development of a Natural User Interface (NUI) for lower limb prosthesis design. The proposed solution exploits the Leap Motion device to emulate traditional design tasks manually performed by the prosthetist. We first illustrate why hand-tracking devices can be adopted to design socket of lower limb prosthesis using virtual prototyping tools. Then, we introduce the developed NUI and its features mainly with regards to ergonomics and ease of use. Finally, preliminary tests are illustrated as well as results reached so far.