Abstract: Astudy on the interaction of Lanthanum and Copper with Alizarine Fluorine Blue has been made. It has been found that La and Cu react with the reagent in a molar ratio of 1:3:3. This ratio was obtained by the improved slope ratio method and modified limited logarithm method. The Forming mechanism of the heteronuclear complex was preseantd.At pH4.0 a red-violet form with maximum absorption at 550 nm is prevalent conditions for the formation of the colour complex were considered. The heteronuclear complex seems to be a more sensitive and accurate system for the determination of copper than the single and isopolynuclear ones.