Electron beam irradiation with moderate fluences of approximately 1016-1017 electrons per cm2 is used for controllable, bulk-scale cutting of single-walled carbon nanotubes (SWCNTs). The effectiveness of high energy electron irradiation in cutting SWCNTs is dependent on the nature of the sidewall. While pristine nanotubes are very stable under irradiation conditions, ozonated SWCNTs combined with a moderate fluence of electrons resulted in bulk-scale cutting of nanotubes. The length distribution of the cut SWCNTs could be controlled by adjusting the irradiation fluence. The average length of the cut nanotubes was 65 nm with 85% of the nanotubes shorter than 100 nm.