We report the thermoelectric properties of thick bismuth-telluride (Bi2Te3) films deposited on polyimide substrates by DC magnetron sputtering and annealed at various temperatures (150–350 °C). The influence of annealing temperature on the microstructure and electronic structure of thick Bi2Te3 films is discussed. In this work, the annealed film at 250 °C has the best thermoelectric property due to highest electrical conductivity and Seebeck coefficient. The main effect of annealing temperature was really helpful to improve crystalline structure and enhance carrier mobility, whereas the carrier concentration was reduced due to the volatile of tellurium atom during annealing. Chemical states of bound and unbound atoms (Bi, Bi3+, Te, and Te2‐) on the surface play an important role in electrical properties. The exceed temperature caused the micro-crack formation and affect carrier transport by the scattering. The power factor of Bi2Te3 deposited by DC magnetron sputtering and annealed at 250 °C is comparable to the power factors of thick Bi2Te3 film deposited by various deposition techniques. The output power of single-leg, thick, thermoelectric Bi2Te3 film annealed at 250 °C as a function temperature generated a power of 0.98 μW at a temperature difference of 50 °C. •Bi2Te3 film of more than 10 μm in thickness was deposited by magnetron sputtering•A maximum PF of 1.35 mW/K2m was obtained following annealing at 250 °C•Annealing treatment improved output power of Bi2Te3 thick film