To evaluate the aging behavior of medium density polyethylene (MDPE) under aggressive environment, principal component analysis (PCA) method was used to establish a nondimensional expression Z from a data set of multiple properties of MDPE. Natural exposure tests of MDPE were carried out for one year with different test periods at Xisha islands in China. Surface properties and mechanical properties of MDPE were measured. It was found that Z increased stepwise with exposure time extending. Combined with the climatic data in Xisha islands, the shielding effect of the oxidation products generated by the degradation process or the reactivating effect resulting from changes of weather conditions were alternatively executed as the determinant factor to affect the change tendency of Z. In a word, Z can be used to reveal the comprehensive degradation pattern of MDPE in natural environments in a reasonable way. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012