Abstract: A simple method is described to functionalize the surface and to modify the structures of multiwalled carbon nanotubes (MWCNTs) grown on silicon substrates using chromium trioxide (CrO3) solution. Unlike nitric acid (HNO3) used in the conventional posttreatment for MWCNTs, the chemical reaction with CrO3 involves amorphous carbon and the carbon nanotubes themselves. It is expected that the surface morphology, chemical composition, and field emission of MWCNTs should be significantly changed after CrO3 solution treatment. The results showed that after 20min of CrO3 solution treatment, the emission currents were enhanced compared with the asgrown MWCNTs. However, extended treatment over 20min was found to degrade the field emission properties of the film. The enhancement in field emission after 20min of CrO3 solution treatment can be ascribed to the modification of surface morphology, the increase in surface density of MWCNTs, and the lowering of the work function. Prolonged CrO3 treatment dissolves MWCNTs and thus results in a decrease in field emission.