Abstract: In this work, the effect and mechanism of causticized starch on flotation separation of magnetite and phlogopite minerals under dodecylamine flotation system were studied. The flotation tests of magnetite and phlogopite were carried out with the collector of dodecylamine, and the depressants of causticized cationic etherified starch (CCES), causticized sodium carboxymethyl starch (CCMS) and causticized dextrin (CD). The interaction way of causticized starch with minerals was analyzed by many methods such as single mineral flotation experiment, Zeta potential analysis, adsorption amount measurements, infrared spectrum analysis (FTIR), and synthetic mixed minerals flotation experiment. The results showed that the difference of surface electrical between phlogopite and magnetite was small, resulting in a poor selectively of dodecylamine for two minerals. All of CCES, CD and CCMS could adsorb on the surfaces of magnetite and phlogopite. The adsorption of the causticized starch on the magnetite surface was dominated by chemisorption, followed by electrostatic adsorption and hydrogen bonding. However, the adsorption of causticized starche on phlogopite surface is electrostatic adsorption and hydrogen bonding. The inhibitory strength of three causticized starches to magnetite followed the order of CCES> CD> CCMS. However, the three causticized starches had poor inhibitory effect on phlogopite.