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Thermoelectric (TE) materials can convert waste heat into electrical energy, which has attracted great interest in recent years. In this paper, the effect of biaxial-tensile strain on the electronic properties, lattice thermal conductivity, and thermoelectric performance of <i>α</i>-phase Se₂Te and SeTe₂ monolayers are calculated based on density-functional theory and the semiclassical Boltzmann theory. The calculated results show that the tensile strain reduces the bandgap because the bond length between atoms enlarges. Moreover, the tensile strain strengthens the scatting rate while it weakens the group velocity and softens the phonon model, leading to lower lattice thermal conductivity <i>k</i>_l. Simultaneously, combined with the weakened <i>k</i>_l, the tensile strain can also effectively modulate the electronic transport coefficients, such as the electronic conductivity, Seebeck coefficient, and electronic thermal conductivity, to greatly enhance the <i>ZT</i> value. In particular, the maximum n-type doping <i>ZT</i> under 1% and 3% strain increases up to six and five times higher than the corresponding <i>ZT</i> without strain for the Se₂Te and SeTe₂ monolayers, respectively. Our calculations indicated that the tensile strain can effectively enhance the thermoelectric efficiency of Se₂Te and SeTe₂ monolayers and they have great potential as TE materials.