For developing age-hardenable, free-cutting aluminum-copper (Al-Cu) alloys that are environmentally friendly, the influence of silicon (Si) addition for chip breaking during lathe cutting was examined in comparison with the addition of conventional elements, lead and bismuth (Pb+Bi). Both Si and Pb+Bi increased the degree of chip breaking; in maximum, 10 times for the Si addition and 100 times for Pb+Bi addition. The increase in the chip breaking frequency due to Si addition was ascribed to the embrittlement of the alloy; Si particles act as crack initiation sites and accelerate crack propagation in the chip. However, chip breaking mechanism for the Pb and Bi added alloy was different; the lake face is covered with a thin layer of Pb and Bi and chips move on the rake face smoothly, leading to severe chip curling. A high performance of chip breaking due to Pb and Bi addition was attributed to resultant, large bending deformation that the chip receives