Abstract: The floatability of hematite and quartz were investigated using dodecylamine as collector and cassava starch, carboxymethyl starches with degree of substitution (DS) of 0.026 and 0.21, and phosphate ester starches with DS of 0.0065 and 0.055 as depressants. The effect of DS on the depressing behavior of modified starches was studied. The flotation results show that native starch, carboxymethyl starch with DS of 0.026, and phosphate ester starch with DS of 0.0065 can strongly depress hematite, whereas carboxymethyl starch with DS of 0.21 and phosphate ester starch with DS of 0.055 weakly depress it. The native starch and carboxymethyl starch with DS of 0.026 can strongly depress quartz, whereas the other three starches are investigated possessed less depressing capabilities for quartz. The results reveal that a low-DS phosphate ester starch is a highly selective depressant in the reverse flotation of silica-containing iron ore with cationic collectors. The Zeta potential measurements exhibit that modified starches with higher DS can lead to lower Zeta potentials of minerals. Thus, the weakened depressing capabilities of modified starches might adsorb cationic collectors on the mineral surface via electrostatic attraction with negatively charged groups stretching into the solution.