The copper/paraffin nanocomposites were synthesized by high energy ball milling. Observation of the microstructure, comparison and ascertainment of the particle size of the composite were analyzed by transmission electron microscope. Then the composite was put into a home-made copper tube (5 mm in diameter, 20 mm in length, closed at one end) by thermoforming process, and its thermal sensitivity was measured using thermo-controlled optical micro measuring equipment. The effects of milling time on the microstructures and the thermal sensitivity of copper/paraffin nanocomposites were investigated. The results showed that different milling time caused the changes in particle size and shape of the composite, and had a significant effect on its thermal sensitivity. With increasing milling time, the thermal sensitivity of composite gradually increased.