The main objective of this work is to investigate human depth perception in virtual reality (VR). Specifically, we attempt to give a first step that towards finding the relationship between size-distance in depth perception in virtual environment. Depth perception has been shown to be key element and a major factor either for simple navigation tasks or for complex and dexterous manipulation tasks. However, in the history of psychology of perception few matters have been of more continuous interest than the issue of the relationship between perceived size and perceived distance. Most studies focused on such questions have converged upon a hypothesis named Size-Distance Invariance. This hypothesis is often stated in the following terms: "A retinal projection or visual angle of given size determines a unique ratio of apparent size to apparent distance" . The relationship between distance and size perception remains unclear in a virtual environment. The effectiveness of virtual environments has often been linked to the sense of presence that users feel in the virtual world. Moreover, Depth perception is one major factor among many believed to underlie presence for teleoperation and virtual environments. Our findings suggest that the strategy based on imagination of motor tasks could have a major effect on users' accurate depth perception in virtual reality.