Hydrophilic microporous membranes were prepared via the thermally induced phase separation process using three types of hydrophilic ethylene-acrylic acid co-polymers (EAA) with different co-unit contents. The effects of diluents on the structure of membranes were also investigated. The experimental results showed that the phase diagrams of these polymer diluent systems were typical diagrams of semi-crystalline polymer diluents with weak interaction. The cloud-point curve shifted to the lower temperature, while the crystallization temperature was not influenced so much by the type of diluents. The pore sizes of EAA membranes prepared from different diluent systems at 30 wt% polymer concentration were about 4.98 mu m, 1.80 mu m, 1.72 mu m, 1.57 mu m, and 0.59 mu m for the systems of EAA3003/diphenyl ether (DPE), EAA3003/soybean oil, EAA3003/dioctyl phthalate (DOP), EAA3003/sesame oil, and EAA3003/peanut oil, respectively. The microstructure of EAA3003-soybean oil system was leafy, while other systems were cellular. Pore sizes and porosities of three EAAs with different co-unit contents decreased in the order of EAA1410/DOP, EAA3002/DOP and EAA3003/DOP.