The anisotropic microporous iPP membranes were prepared via thermally induced phase separation, by using a novel codiluent that is a mixture of dibutyl phthalate (DBP) and soybean oil. A polymer concentration gradient cross thickness was formed by evaporating the codiluent from top of membranes prior to phase separation and an anisotropic structure of membrane was obtained after solidification. The effects of thickness of membranes, atmospheric temperature, temperature at bottom of membranes and evaporation time on the membrane structure were investigated by scanning electron microscopy (SEM). Furthermore, pore sizes were measured to analyze anisotropy of membranes. Eventually, an optimized condition for deriving apparent anisotropic membranes was found.