The used material for battery separator membranes for automotive applications is generally based on Polyethylene. These PE separators are porous membranes composed of a blend of ultrahigh molecular weight polyethylene (UHMW-PE), naphthenic oil and precipitated silica. During the elaboration process, the membrane is formed at a temperature higher than the melting point of UHMW-PE and then the material is cooled. This cooling generates two phenomena, the UHMW-PE crystallization and a phase separation between UHMW-PE and naphthenic Oil. The membrane’s porosity is finally obtained by extraction of the naphthenic oil. With the aim of understanding the membrane structuring mechanisms, we studied the influence of silica and oil concentrations on the UHMW-PE crystallization. Blends with polyethylene/precipitated silica/naphthenic oil were performed in an internal mixer and analyzed by Differential Scanning Calorimetry (DSC) and by Thermogravimetric Analysis (TGA) to study UHMW-PE crystallization. Three kinds of polyethylene have been studied, UHMW-PE, HDPE (High Density Polyethylene) and L-LDPE (Linear Low Density Polyethylene). Results have shown that, for each polyethylene, the crystallization temperature is controlled by the Oil/PE ratio. Moreover, only crystallinity of UHMW-PE is controlled by the Oil/UHMW-PE ratio of the blend. Crystallinity of UHMW-PE varies from 40% to 65% and the initial high crystallinity (χc≈65%) of UHMW-PE can be recovered in the membrane with a high Oil/UHMW-PE ratio in the membrane. Furthermore, precipitated silica indirectly contributes in increasing the UHMW-PE crystallinity by increasing the possible oil content in the blend.