Structural control, and especially chirality control, remains a significant challenge in the synthesis of single-walled carbon nanotubes (SWNTs). We report herein a rational approach to engineering fullerene caps for growing SWNTs with controlled structures via chemical vapor deposition (CVD). Opening of fullerendione via thermal oxidation yields hemispherical caps which can initiate SWNT growth at their open ends. The size and structure of these caps can be engineered by tuning the temperature of thermal oxidation. Results show that pregrowth treatment of the cap is indispensable to successful growth of SWNTs. The temperature used for thermal oxidation strongly affects the size and structure of the cap and further determines the diameter distribution of the as-grown SWNTs. Stronger oxidation treatments (450 degrees C oxidation in air) promote formation of thinner SWNTs, while weaker oxidation treatments (350 degrees C oxidation in air) lead to wider SWNTs. Interestingly, SWNTs made using fullerene caps show steplike diameter distributions relative to SWNTs catalyzed by Fe nanoparticles. This cap engineering using opened C(60) provides a potential approach to grow SWNTs with controlled structures.