Poly(vinylidene fluoride) (PVDF) flat membranes were prepared via thermally induced phase separation (TIPS) process using mixed diluent of dibutyl phthalate (DBP) and dioctyl adipate (DOA). The properties and structure of the porous membrane were characterized by field emission scanning electron microscope (FESEM), porosity, flux test, contact angle test, mechanical properties test and differential scanning calorimetry (DSC). The effects of PVDF concentration and mixed diluent composition on the structure and properties of the membranes were investigated. The results show that PVDF/DBP/DOA systems in the membrane formation process are mainly S-L phase separation, which show spherulites structure. With increase of the PVDF concentration and DOA ratio in mixed diluent, the size of membrane cross-sectional spherulites structure reduces, the membrane surface porosity is reduced and the density is increased. In addition, as the DOA ratio in mixed diluent increases, the mechanical properties change better, but the flux decreases.