Abstarct: Recently, the design of a battery with almost permanent performance and suitable for electronic circuits has received much attention to meet the needs of today's advanced circuits. This is possible through harvesting energy from ambient sources via boost converters. Among these energy sources, the human body heat energy is very suitable for wearable applications due to the stable temperature difference between the human body and the environment. But since the energy harvested from the human body is low, the main challenge of these converters, is to improve the low input voltage and the maximum efficiency. Traditional boost converters often use discontinuous conduction mode (DCM) due to the low switching loss of this operating mode. In low input voltages, the total loss of the DCM is less, however at higher voltages, the conduction loss prevails over the switching loss, and the continuous conduction mode (CCM) will have lower losses and can be more efficient. For this reason, in contrast to the Traditional boost converter for thermal energy harvesting, the proposed converter uses both CCM and DCM operating mode to simultaneously improve the minimum input voltage and maximum efficiency at high input voltages. This is accomplished by a low-power digital operating mode detection circuit, which switches between the two operating modes at a specific input voltage obtained by analytical solutions. The suitable boundary voltage equation for changing the operating mode is presented baxsed on the values of boost converter circuit elements. Finally, the proposed boost converter is implemented in a 0.18 µm CMOS process using Cadence Virtuoso software. The simulation results show that the peak efficiency of the proposed boost converter can be up to 94%, 60.4% and 35% at 300, 20 and 15 mV of the open-circuit voltage, respectively.