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The structural, electronic, and thermoelectric properties of Ba1−xSrxZn2Sb2 (x = 0, 0.25, 0.5, and 0.75) are studied by the full-potential linearized augmented plane wave (FP-LAPW) method in the density functional theory (DFT) framework. The band structure and density of states of the studied system are calculated using PBE functional and Tran-Blaha modifed Becke-Johnson (mBJ) exchange potential. Our
generalized gradient approximation (GGA) results show that the system exhibits a phase transition from the orthorhombic phase to the trigonal one at x = 60%. With the TB-mBJ potential these compounds are narrow-gap semiconductors with a direct band gap for x = 0 and 0.25, whereas an indirect band gap semiconductor for x = 0.5 and 0.75. The thermoelectric properties are calculated using the semi-classical Boltzmann transport theory. The temperature dependence of thermoelectric transport properties of these compounds is discussed and compared with available experimental data. Substitution of Ba by Sr leads to an increase in the figure of merit (ZT). The Seebeck coefficients and ZT of Ba0.75Sr0.25Zn2Sb2 and Ba0.5Sr0.5Zn2Sb2 are even larger than those of BaZn2Sb2. The predicted highest ZT values are found for x = 0.5 at higher temperature.