tIn this paper, a plasma jet driven by an in-house developed microsecond pulse is used for polymethylmethacrylate (PMMA) surface modification. The hydrophilic modification effects of He and Ar plasma jetsare compared under the same condition. The He and Ar plasma jets are characterized by optical emissionspectrometer (OES). Water contact angle (WCA) measurement is used to evaluate the wettability ofPMMA samples. The evolution on morphology and chemical composition of PMMA before and afterplasma treatment are also analyzed. The OES results demonstrate that He plasma is composed withhigher intensities of reactive species, like OH, O, N2and N2+than that of Ar plasma and show a bettermodification effect. In addition, it is observed that the surface roughness and oxygen-containing groupslike C O/C OH and O C O increase on the PMMA surface after plasma treatment, which are responsiblefor the hydrophilic modification. During the storage, the WCA of each sample increases gradually for bothHe and Ar plasma treatments. The He plasma treated PMMA also shows a slower aging effect than thatof Ar plasma treated PMMA.