Effects of an external transverse electric field on adsorption and diffusion of Li atoms on the single-walled carbon nanotubes (CNTs) were investigated using density functional theory. Results showed that the adsorption energy was significantly enhanced by applying the electric field. As the external electric field was increased from 0.0 to 0.6 V/Å, the adsorption energies were decreased from -1.37 to -2.31 eV, -1.32 to -2.46 eV and -1.33 to -2.63 eV for the Li atoms adsorbed on (6,6), (8,8) and (10,10) CNTs, respectively. Meanwhile, the diffusion barriers of the Li atoms on the CNTs were also decreased as the external electric field was applied. When the external electric field was increased from 0.0 to 0.6 V/Å, the energy barriers were decreased from 0.42, 0.40 and 0.39 eV to 0.20, 0.17 and 0.15 eV for the Li diffusion in the (6,6), (8,8) and (10,10) CNTs, respectively. The results proved that an external electric field can be applied to enhance the adsorption and diffusion of Li atoms on the CNTs (used as the anode) for lithium ion batteries.