The polypropylene (PP) microporous membrane has been used in the Lithium battery as a separator. During the charge and discharge process, the temperature in the battery may rise. To clarify the microstructure change mechanism of membrane during the temperature increase, the PP microporous membranes were heat treated at different temperatures and the microstructure and properties were characterized using differential scanning calorimetry, scanning electron microscopy and atomic force microscopy. When the membranes were treated at temperatures lower than 130 degrees C, no apparent dimension and microstructure changes were observed. The Gurley value (characterizing the air permeability) was kept around 240 s. When the membranes were treated at temperatures higher than 140 degrees C, apparent dimensional shrinkage and the decrease or even disappearance of connecting bridges appeared. The corresponding Gurley value was increased to 1320 s, indicating worse air permeability property. The final heat-setting stage during the preparation of microporous membrane based on melt-stretching mechanism could stabilize the stretched connecting bridges and when the heat-treatment temperature was higher than heat-setting temperature, the crystals formed during heat setting could not support the stretched connecting bridges and resulted in the retraction or even disappearance of connecting bridges. These retracted connecting bridges were converted to new crystalline nodes sitting on both sides of separated lamellae.