The morphological changes and improvement of membrane properties caused by heat treatment were investigated for polytetrafluoroethylene (PTFE) porous membranes prepared from a fine powder by extrusion, rolling and stretching. The properties of the membrane were significantly changed by heat treatment at temperatures higher than 320 degrees C. Shrinkage diminished and the mechanical strength increased due to the partial melting of PTFE. The increase in mechanical strength was caused by suppression of new fibril formation as a result of the loss of folded ribbon-like crystalline structures that facilitated fibril structures to be pulled out of the original PTFE particle. The decrease in shrinkage was caused by the transformation of fibrils, formed as a collection of ribbon-like structures, into a massive fibrous structure, which inhibited the reformation of particles. The most important change of the porous structure caused by the heat treatment was the union of nodes in the direction of stretching resulting in a PTFE porous membrane with larger spatial periodicity. A heat treatment above melting temperature of PTFE was the most effective. However, it was necessary to control the temperature and time in order to restrict the coarseness of the porous structure of the membrane.