Generation of geometric shapes called "geometric concept designs" via the multimodal user interface of a virtual reality (VR) based system motivates the current research. In the VR system, geometric designs can be more effectively input into the computer in a physically intuitive way (using voice, hand motions, and gestures-just as a person would communicate ideas to another person). Although the dimensions inputted may be imprecise, this approach is useful for concept shape generation since at this stage dimensions need not be exact. The paper focuses on determining the requirements for the multimodal user interface and assessing the applications of different input and output mechanisms in the virtual environment (VE). Based on the results of these investigations, a VR-based computer-aided design (CAD) system for the conceptual shape design will be implemented in the current research. Once designers use the system that we are developing, the interface requirements determined in the current paper may be verified or refined. The objectives of the research are to expand the frontiers of product design and establish a new paradigm for the VR-based conceptual shape design system.