The structural integrity of the separator is crucial to the abuse tolerance of a battery. To estimate its stress level in a battery, the mechanical property of the separator in situ in the battery environment must be known. This work investigated the tensile behavior of a single layer polypropylene (PP) separator in electrolyte solutions for Li-ion batteries using a dynamic mechanical analyzer (DMA). The measurements were carried out in both dry (ambient) and wet conditions for both the machine direction (MD) and the transverse direction (TD). In the wet condition, samples were submerged either in a DMC solvent or in a electrolyte solution of 1.1M LiPF6 in EC/DMC (1/1 by volume). The DMA experiments were performed under uniaxial tension, creep, and frequency sweep modes. The results in all three modes demonstrated that the mechanical properties of the separator were significantly lower in wet conditions. For instance, in the MD, relative to the dry condition, the ratio of the Young's modulus was about 0.49 and 0.52 for DMC and 1.1M LiPF6 in EC/DMC, respectively. The results indicate that the mechanical properties measured in dry condition using samples that had been preconditioned in solutions are not sufficient to represent the in situ material behavior.