The evolution of structure and properties of poly(p-phenylene terephthalamide) (PPTA) with different hydrothermal aging temperatures were systematically investigated. The cooperative change in tensile strength and reduced viscosity upon aging time indicated a direct chemical structure-property correlation. The linear relationship between tensile strength and reduced viscosity was explored. Wide angle X-ray diffraction measurements disclosed the crystal structure of aramid fibers was stable with aging time and temperature. Fiber exhibited skin and core structure in different ways. Cracks and microfilbrils were observed in the core after hydrothermal aging, while the structure was unchanged in the skin. Fourier transform infrared spectroscopy also proved that the chemical structure of the skin was not affected by degradation. The results elucidated the hydrolysis of amide function happened in amorphous or bundles between microfibrils in the core rather than in skin of the fiber. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012