Fluorimetric method has been proposed for the speciation of Al in natural waters, based on the different competitive complexation behavior of analytical reagent and natural organic matter to Al. With eriochrome blue black R (EBBR) or morin, the monomeric inorganic Al (Al(i)) was determined, while with 8-hydroxylquinoline, the total monomeric Al (Al(a)) was obtained. The analytical results of Al speciation in natural water are close to those by Driscoll's method. The method is simple and reliable. Fractionation of Al was realized by selecting appropriate reagents under the pH of natural water determined. The merits of this novel strategy are that the disturbance to original equilibrium of Al species is minimized and the results conform to the actual situation of natural waters.