In some phenomena of visual perception, the visual system shows different behavior in oblique directions from the cardinal (vertical and horizontal) directions, and it is called “Oblique effect”. In this study, we investigated the oblique effect of vection strength. Random dot optical flows which moved in 16 directions (0, ±20, ±45, ±60, ±90, ±120, ±135, ±150, 180°) on the frontal parallel plane were used as stimuli, and vection strengths (latency, duration and magnitude) were measured for each direction. The results significantly indicated that vection strength in oblique direction was weaker than the cardinal directions.