alpha-MgAgSb is a promising thermoelectric materials having good performance at medium temperature. Native defects in alpha-MgAgSb are frequently reported experimentally and are tightly involved in the thermoelectric properties of alpha-MgAgSb. In this paper, all possible native defects in alpha-MgAgSb are calculated as well as detailed results are given and discussed. The concentrations of several dominant native defects, for example, V-Ag and A(gsb), could reach up to 10(-4) cm(-3) at 540 K. Furthermore, the electronic structure and transport properties of alpha-MgAgSb with dominant native defects are investigated. Results show that the introduction of Ag-Mg and V-Ag contributes to a much lower inertial mass and slight decrease in Seebeck coefficient. The lattice thermal conductivity is greatly reduced with the introduction of native defects. For alpha-MgAgSb with V-Ag, the peak ZT could reach up to 1.84 at 420 K. Our calculation demonstrates that defect engineering is an effective strategy to enhance thermoelectric performance of the materials. (C) 2017 Elsevier B.V. All rights reserved.