Fluorescent intensity of cotton fabric whitened to the maximum degree with two typical stilbene type fluorescent brightening agents (FBA) gradually decreased to zero for 500 hr light irradiation, and both visual whiteness and psychometric coordinates a ^* b ^* returned to that of unwhitened original fabric. a ^* b ^* coordinates of irradiated fabric in CIE color diagram retraced the loci found for the whitened fabric with increasing FBA concentration. Change in the pattern of fluorescent emission spectrum by the irradiation revealed the decrease in trans-isomer fraction of FBA on the fabric. Decay within a short time was larger for the irradiated upper surface than the back surface, and the difference between the two surfaces was proportional to the thickness of the fabric. Over-dyed fabric exceptionally increased in the whiteness within a short time, which may be related to the decrease in the reabsorption of the fluorescence, caused by cis-isomerization of the excess FBA on fiber surface. Considerable yellowing was recognized on over-dyed fabric after 500 hr exposure, suggesting the formation of carbonyl derivatives by photooxidation.