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p-type YbCd 2-x Mg x Sb 2 (x = 0.1, 0.2, 0.4, 0.6, 0.8) Zintl phases have been synthesized via melting reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering (SPS) densification processes. The effects of isoelectronic substitution of Mg for Cd on thermoelectric properties have been investigated. Electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been performed as a function of temperature from 300 to 650 K. The XRD patterns show that the solubility limit x of Mg in the YbCd 2-x Mg x Sb 2 is less than 0.8. With increasing Mg content x, the cell parameters a and c of the YbCd 2-x Mg x Sb 2 compounds decrease. The isoelectronic substitution of Mg for Cd leads to a remarkable decrease in total thermal conductivity and an increase in Seebeck coefficient, though a slightly decease in electrical conductivity. As a result, the maximum dimensionless figure of merit ZT of 1.08 is obtained at 650 K for YbCd 1.9 Mg 0.1 Sb 2 . Compared with the ZT of 0.78 at 650 K in the unsubstituted YbCd 2 Sb 2 , it is improved greatly by 38%.