This study investigates the fabrication and characterization of an energy harvesting device using the commercial 0.18 µm CMOS (complementary metal oxide semiconductor) process. Based on the thermoelectric method, the energy harvesting device converts thermal energy into electrical power. The energy harvesting device is constructed by 408 thermocouples in series, and each thermocouple is composed of p-type and n-type polysilicon strips. In order to increase the temperature difference in the hot and cold parts of the thermocouples, the hot part of the thermocouples is suspended to reduce heat sink. The experimental results showed that the energy harvesting device had an output voltage of 0.4 mV at the temperature difference of 15 K and an output power of 125 nW at the temperature difference of 15 K.