Influences of particle sizes on the discharge performances of microsized pyrite has been investigated, 15µm ~30µm 50µm ~75µm and 100µm ~150µm pyrite samples were manufactured. Single cell thermal batteries were fabricated for microsized pyrite evaluation. The cells were tested at 400 ^oC and 500 ^oC under baseline loads of 0.2 A/cm² and 0.5 A/cm² respectively, to evaluate the influences of temperature on the electrochemical performance of a thermal battery. Pulse current discharges were also used to test the discharge performance of the single cell thermal battery with different pyrite particle sizes. The results showed that cells with smaller pyrite particles had higher voltage plateau and larger specific capacities, were able to sustain higher current densities than cells with larger pyrite particle sizes. The pulse current test under 400 ^oC showed that the cell cannot afford the current, and the 15µm ~30µm pyrite thermal battery own the largest discharge current, which may be due to the large contact between the pyrite and the electrolyte. 8-cell thermal batteries were used to conduct the discharging test at 0.2 A/cm² 0.5 A/cm² and pulse current conditions, which revealed that battery with 15µm ~30µm pyrite cathode had higher votage plateau and larger specific capacity than the 100µm ~150µm ones. At 0.5 A/cm² discharge conditions, the power densities of the 15µm ~30µm pyrite thermal battery stacks is 12.5% higher than that of the 100µm ~150µm ones. Through the investigation of the electrochemical performance of the 15µm ~30µm pyrite thermal battery, they are suitable for thermal batteries discharging at large current densities.