Abstract: To optimize the thermal resistance of thermoelectric generator for improving performance, the output performance variation of thermoelectric generator with different thermal resistance at both sides was investigated. The external ceramic substrates with different thermal conductivity and thickness were used to simulate the thermal resistance at both sides of thermoelectric generator. The variation of temperature difference, open-circuit voltage, output power and maximum output power of thermoelectric generator at different hot source temperatures and load resistances was analyzed. The results show that when the thermal resistance at both sides of the thermoelectric generator is the same, the temperature difference and the open-circuit voltage are increased with the increasing of hot source temperature. When the thermal resistance at both sides of thermoelectric generator is different, the open-circuit voltage and the maximum output power are increased with the increasing of hot source temperature. The effective temperature difference is increased with the increasing of load resistance, and the power is increased first with latter decreasing when the load resistance is increased. Compared to the performance of thermoelectric generator with the same thermal resistance at both sides, the ceramic substrate with smaller thermal resistance at the hot side is more beneficial to improve the output performance of thermoelectric generator, and the improvement effect is more obvious near the maximum power point. The open-circuit voltage, the maximum output power and the maximum effective temperature difference can be increased by 0.64 V, 0.87 W and 24.5 ℃, respectively.