A plasma jet driven by microsecond pulse was used for polymethylmethacrylate (PMMA) surface modification. In the paper, the effect of different proportions of Ar and CF4 gas mixture on PMMA modification was studied. After plasma treatment, PMMA samples were immersed in water to accelerate the aging effect. The morphology and elemental composition of the material surface were investigated by atomic force microscopy and X-ray photoelectron spectroscopy. In addition, the mechanism for plasma surface modification has been analyzed. Results showed that plasma treatment introduced hydrophobic groups (i.e. C-F-n and C-F) into material surface and increased their surface roughness. In the initial stage of plasma treatment, the contact angle of PMMA decreased and showed a hydrophilic property. After water immersion with different time, the oxygen-containing groups (C=O) were gradually displaced by C-F-n and C-F and showed a hydrophobic property. The increased roughness and fluoride groups on the PMMA surface were responsible for hydrophobic behavior of PMMA. (C) 2014 Elsevier B.V. All rights reserved.