Multiwalled carbon nanotubes (MWCNTs) were irradiated by high-energy electron beams with different dosing amounts, and the physical properties including morphology and local surface structure were investigated by using a gas adsorption isotherm apparatus. The layering properties of argon on MWCNTs were measured from 65 to 80 K, and the interaction of argon on the surface was analyzed. Little change of surface structure between unirradiated and irradiated MWCNT samples was found. Interestingly, broader isotherm steps from the electron beam irradiated samples were found, although the amount of gas molecules forming the first atomic layer remains the same for the samples before and after irradiating the beams. This observation was supported by calculated values of the two-dimensional compressibility. Our combined results suggest that dosing of the electron beam on the carbon nanotubes induced the local surface defects, while no structural modification occurs.