Human-robot interaction (HRI) takes place espe-cially through interfaces. The design of such interfaces is avery delicate and crucial phase because it influences the robotaccessibility and usability by the user. In this paper, we describeand analyze the results of 2 tests conducted so as to understandsome of the optimal features that should characterize the robotvoice and graphical-based user interfaces. Our test platform isan assistive robot developed for the elderly with mild cognitiveimpairments. Therefore, the user interfaces must be clear andsimple. The ambiguities must be eliminated so as to facilitatethe use of the robot and hence not to discourage the elderlypopulation to use new technologies.