The aging effects of atmospheric plasma treatments on UHMPE fibers are studied. UHMPE fibers are treated for 0.5 and I min with He/O-2/air gas and for 2 and 4 min with He/air gas by atmospheric pressure plasma on a capacitively coupled device at a frequency of 5 kHz. The samples are tested for fiber/epoxy interfacial shear strength at time intervals of 0, 3, 15 and 30 days after initial plasma treatment. Scanning electron microscopy shows micro-cracks on each set of treated fibers, which is not affected by aging over the 30 day study. Interfacial shear strengths (IFSS) for plasma-treated fibers are 2-3 times as high as that of the control. The IFSS for the plasma treated fibers remains constant up to 15 days and then decreases afterwards. XPS Analysis shows a slight increase in atomic concentration of oxygen and nitrogen for each plasma-treated sample. For the He/O-2/air plasma-treated samples, XPS analysis shows an observable increase in C-OH bonds, C=O bonds and COOH bonds, while for the He/air plasma-treated samples, there is a slight increase in C-OH and O=C-O bonds. After 30 days, a decrease in oxygen content for all plasma-treated samples is manifested.