Mixed reality (MR) is a technology that merges real and virtual worlds in real time. In MR environments, visual appearance of a real object can be changed by superimposing a virtual object on it. In this study, we focus on the center-of-gravity (COG) and verify the influence of MR visual stimulation on the COG in MR environments. This paper describes the systematic experiments performed to study the influence. The results obtained are interesting: (1) the presence of COG can be changed by MR visual stimulation; (2) although COG differs in vision and force, the presence of COG can be represented by MR visual stimulation under certain conditions; (3) COG perception can also be changed by varying the mass of the real object. We named this psychophysical influence the “Shape-COG Illusion.”