N, N-diethyl dodecyi amine(DEN12) was synthesized from dodecyl amine, formic acid and acetic aldehyde. The collecting property of DEN12 on diaspore, kaolinite and illite was investigated by flotation test and infrared spectrum. The results show that in the presence of 2.0×10-4 mol/L DEN12, the recoveries of kaolinite and illite are all higher than 78% and the recovery of diaspore is 50% in the pH range of 5.5-6.0. The mass ratio of Al2O3 to SiO2 in concentrate obtained from separation artificial mixture is higher than 10, suggesting that DEN12 can be used as a collector to separate the aluminosilicates from diaspore in bauxite ores at the pulp pH below 8. The measurements of the infrared spectrum approve that the action between ahiminosilicates and tertiary amine collector is strong electrostatic adsorption and that of diaspore is weak electrostatic adsorption.