Abstract: In a buffer medium of pH 10,fluorescence of the reagent o-phenanthroline was enhanced due to the formation of a complex with Mg2+ ion,when measured at the wavelengths of 235 nm (λex) and 360 nm (λem).The magnitude of increase in fluorescence intensity (ΔF) was found to keep linear relationship with concentration of Mg2+ ion in the range from 0.1-2.0 mg·L-1.Detection limit (3s/b) found for the method was 0.038 mg·L-1.Three water samples of different sources were analyzed for their magnesium contents by this method,values of RSD′s (n=6) found were in the range from 1.3%-1.6%.Test for recovery was made,giving values of recovery ranged from 98%-103%.