Surface modification of poly(methylmethacrylate) (PMMA) substrates has been demonstrated using an excimer lamp radiating vacuum ultraviolet (VUV) light of 172 nm in wavelength. In this study, we have particularly focused on the effects of atmospheric pressure during VUV irradiation. Each of the substrates was photoirradiated with VUV light under a pressure of 10, 103 or 105 Pa. Changes in nanomechanical properties of the VUV-irradiated sample surfaces were studied based on a scratching test using a nanoindenter. The wear-depth of the PMMA sample treated at 105 Pa was about 137.0 nm, which was much larger than the wear-depth of an untreated PMMA substrate (63.3 nm). On the contrary, when samples were prepared with VUV irradiation conducted at 10 and 103 Pa, their wear-depths markedly decreased down to about 2.2 and 12.5 nm, respectively. The sample treated at 10 Pa was particularly wear-resistant. This high wear-resistance was attributable to the formation of new carbon-carbon bonds such as C=C bonds on the PMMA surfaces.