This study systematically investigates the uniaxial and biaxial tensile deformation behavior of polytetrafluoroethylene (PTFE) membranes which are used for water purification. The present PTFE membrane has micron size pores with open cell structure, and the pore is anisotropic shape. During the uniaxial tensile test, the membranes undergo elastic deformation and plastic deformation with strain rate sensitivity (i.e. time-dependent deformation behavior). In addition, it strongly demonstrates anisotropic deformation, i.e. deformation behavior is different along longitudinal direction and transverse ones. To clarify the microscopic deformation mechanism, in-situ SEM observation is carried out during tensile loading. It is found that the anisotropic deformation behavior appears due to the inherent pore structure. Next, to investigate deformation behavior under biaxial loading condition, small punch test using a spherical indenter is carried out. The membrane undergoes elastic and plastic deformations. Finally, crack nucleates around the indenter contact and indenter completely penetrates through the membrane. It is also found that the membrane demonstrates anisotropic out-of-plane deformation behavior. To clarify these mechanisms, FEM computation is carried out, such that experimental results of force-displacement curve and out-of-plane deformation behavior are compared with the computational ones. The present FEM model enables the prediction of the membrane’s deformation behavior under biaxial loading.