We have reported a first principles study of structural,mechanical,electronic,and thermoelectric properties of the monoclinic temary thallium chalcogenes Tl2MQ3 (M =Zr,Hf;Q =S,Se,Te).The electronic band structure calculations confirm that all compounds exhibit semiconductor character.Especially,T12ZrTe3 and T12HfTe3 can be good candidates for thermoelectric materials,having narrow band gaps of 0.169 eV and 0.21 eV,respectively.All of the compounds are soft and brittle according to the second-order elastic constant calculations.Low Debye temperatures also support the softness.We have obtained the transport properties of the compounds by using rigid band and constant relaxation time approximations in the context of Boltzmann transport theory.The results show that the compounds could be considered for room temperature thermoelectric applications (ZT ～ 0.9).