Recently, we easily access 3-dimensional (3D) entertainments, i.e. 3D broadcastings or 3D cinemas. Stereoscopic visions in humans use binocular and monocular depth cues such as binocular parallax, vergence, lens accommodation, size of visual object, shading, and so on. Strong depth cues, however, may affect human visual systems and it may induce dizziness or headaches. Then it should be very important to know optimal condition of depth cues and interaction within the cues. In this study, we focused on two depth cues;binocular parallax and object size. We investigated how these cues affect depth perception and the interactions of the cues. We stereoscopically presented two visual stimuli on a 3D TV for subjects and they were required to answer which stimulus was presented nearby them. They answered correctly when viewing normal size condition, i. e. nearer stimulus presented larger than the farther one. On the other hand, their performances dropped when they watched illusion-condition stimulus, i.e. farther stimulus presented larger than the nearer one. These results indicate the object size may one of the important factors for stereoscopic depth perception in the virtual 3D environments. The subjects were divided into two groups according to their performance (cutoff was 50% correct). We found vergence eye movements in the lower score subjects were dysfunctional, especially on convergence. Vergence functions may affect depth perception.