The effect of hydroxamic acid starch (HAS) and hydroxamic acid polyacrylamide (HPAM) on the flotation of diaspore and kaolinite was investigated by flotation test. It is found that HAS depresses diaspore but activates kaolinite in acidic pulp, while HPAM activates both diaspore and kaolinite in the pH range of 2.0–10.5. The measurement of zeta potential shows that both HAS and HPAM can increase zeta potential of negatively charged diaspore, which indicates the existence of chemical bonding or hydrogen bonding between the reagents and diaspore. By covering the collector dodecyl amine (DDA) on diaspore surface, HAS increases the hydrophilicity of minerals and depresses the flotation of diaspore, however HPAM activates the flotation of diaspore by increasing the adsorption of DDA on diaspore surface.