The effect of flotation agent octadecyl amine (ODA) on the complex titration of both magnesium and calcium ions was studied with two groups of comparative experiments： (1) Before titration, the suspension was not filtered. In this case, ODA had a great effect on the complex titration of both magnesium and calcium ions. The titration end-point of magnesium ions was difficult to he determined. Although the titration end-point of calcium ions could be determined, there was an obvious experimental error compared with the blank solution without ODA. These results were confirmed by the UV-Visible spectrum analyses of the related solutions. (2) Before titration, the suspension was filtered. In this case, the influence of ODA on the complex titration of both magnesium and calcium ions could be removed. UV-Visihle spectrum studies showed that, in this case, both the spectra and time scanning curves of the tested solutions were similar to those of the blank solutions.