The mean lives of positrons annihilating in nitric acid treated polypropylene were determined. A long‐lifetime component with a mean life of about 50 nsec appeared when the polypropylene was treated in hot nitric acid for more than three hours. This was obviously due to the annihilation ofortho‐positronium in the microcavities in the polypropylene etched by the action of nitric acid. It is suggested that nitric acid attacks the amorphous part of polypropylene first and leaves a skeleton of crystalline form filled with microcavities. The change of the mean life and the intensity of the long lifetime component with the change of the nitric acid treatment time is discussed. The value of theo‐Ps diffusion coefficient in polypropylene is estimated to beD>4.5×10−4cm2sec−1and the potential of the barrier surrounding theo‐Ps in the cavity is estimated to be less than 1 eV.