The free-volume properties of uniaxially stretched polytetrafluoroethylene (PTFE) have been studied by positron lifetime spectroscopy as a function of residual deformation epsilon in the range of 0-180%. According to the variation of ortho-positronium (o-Ps) intensity, the stretch process can be divided into three stages, i.e. elastic region ( epsilon = 0-20%), plastic-flow region ( epsilon = 20-80%) and strain-strengthening region ( epsilon = 80-180%). In each stage the free-volume concentration is constant, but it has a step increase at the transition points ( epsilon = 20% and 80%). The average radius of free-volume holes increases with deformation in the elastic region, has a maximum in the plastic-flow region, and finally tends to a lower level in the strain-strengthening region. These results show that positron annihilation lifetime is a sensitive means to probe the microstructural change of polymers during deformation.