The spatial characteristic of color assimilation with a grating was investigated in terms of the perceptual size. Strength of assimilation was assessed on color gratings at several vergence angles without changing the retinal size. The perceived size of the grating was also measured as a Junction of the vergence angle. Our results showed that the strength of color assimilation was modulated by perceived size. At a larger vergence angle, the perceived size becomes smaller and the appearance of color stripes shifts toward a spatially averaged color, despite no change in retinal size. This spatial characteristic cannot be explained by any retinal processes, suggesting cortical processes of color assimilation. (C) 2004, Wiley Periodicals, Inc.