Thermal batteries are used for military power sources that require robustness and long storagelife such as missiles and torpedoes. FeS2 powder is currently used for cathode materials because of itshigh specific energy density, environmental non-toxicity and low cost. However, large particle size ofconventional FeS2 has been deterred its possible application for higher power thermal batteries. In order toimprove the power density, high energy ball milling of FeS2 has been introduced to crush the micron-sizedFeS2. Discharge characteristics of the single cells fabricated with nano-materials and conventional FeS2powder were evaluated.