Fluorescent dyestuffs are used according to fashion trends in leisurewear clothes, esp. sportswear and in the work clothes worn outside. They are used mostly today, in coloring of polyester and polyamide fibers by dyeing and printing. A new fluorescent reactive dyestuff in the market gives possibility to apply some colors to cotton fibers by itself and by combination with others. In this research, dyeing of cotton fibers with reactive fluorescent dyestuff and the light fastness values are tried to be determined and developed because they were low in all of the fluorescent dyestuffs. 100% cotton fabrics were used and exhaust dyeing method was applied in the experiments. The fastness values of the dyed fabrics before and after the treatment of UV Absorbers were determined and evaluated. The fastness values of the fabrics were good except than the light fastness values and as the result of the light fastness developing work, a little progress was achieved.