The simultaneous adsorption of poly(amidoamine) dendrimers with surface carboxyl groups and an anionic surfactant, sodium dodecyl sulfate (SDS), on positively charged alumina particles was investigated at pH 5 by measuring the amount of adsorbed dendrimers and SDS, zeta potential, and sedimentation rate of alumina suspensions. The dendrimers of generations G1.5 and G5.5 were used. Under constant feed concentrations of the dendrimers, the amount of adsorbed dendrimers gradually decreased, while SDS adsorption increased with increasing SDS concentration for both SDS/G1.5 and SDS/G5.5 systems, indicating a competitive adsorption between the dendrimers and SDS. A preferential adsorption of G5.5 over G1.5 in the competitive adsorption with SDS was also observed. The difference in the adsorption of the dendrimers reflected the sedimentation rate of alumina suspensions; the feature of the sedimentation rate with SDS concentration for the SDS/G1.5 system was very similar to that for the SDS system alone, but different from that for the SDS/G5.5 system. In addition, the simultaneous adsorption behavior of dendrimers and SDS on alumina was considerably different from that of a linear polymer, poly(acrylic acid), and SDS that the displacement of poly(acrylic acid) on alumina is easily taken place by SDS.