Ion irradiated poly-lactic acid (PLA) was investigated using a slow positron beam. The positron annihilation S-parameters of the ion irradiated samples varied with positron beam energy while those for pristine one were independent of the positron energy. The density and the loss of mass for ion irradiated PLA were obtained using slow positron beam, AFM, SAICAS and STMI measurements. These measurements revealed that the irradiated part was shrunk to about half thickness and the density decreased to about half, so that about 3/4 of mass was lost by the irradiation.