The morphology and its formation has been studied for the steady-rate stretching operation of polytetrafluoroethylene (PTFE) porous membranes, which were prepared from PTFE fine powders with a mean diameter of approximately 2 x 10(2) nm through extruding and rolling operations prior to the stretching operation. The uniaxially stretched membranes were produced by the unique stretching operation parallel to the rolling operation, and the biaxially stretched membranes by the dual operations consisting of the parallel and the subsequent perpendicular stretches. The inversion of the stretching direction, i.e., the first stretching operation perpendicular to the rolling operation and the second one parallel has been observed to be impossible due to the occurrence of macroscopic fractures on the membrane during the first stretching operation. The uniaxially stretched porous membranes are comprised of fibrils completely oriented in the stretching direction and remaining island-like fractures of the rolled PTFE sheet. The second stretching operation provides a lattice-like porous structure by giving the island-like fractures further division along the second stretching direction and the fibrils slant from the original orientation. The stretching operation is possible within the range where the relative elongation of the whole membrane along the second stretching direction is less than 50% of that along the first one, indicating that the fibrils yielded by the first stretching operation sustain the lattice-like porous structure induced by the second stretching operation. The distribution of the slant angle of the fibrils is independent of the elongation in the second stretching operation, thus, the division of the island-like fractures linked with the fibrils steadily proceeds during the second stretching operation.