Positron lifetime measurements were performed on biological samples of lima bean and peanut. The experimental results showed that there were many small holes of diameter 0\^48nm and 0\^7nm, respectively, in the lima bean and peanut samples. The very special micro-hole structure of plant seed is the basis of biological effects caused by ion implantation of low energy. The peanut seeds were implanted with vanadium at an energy of 200keV. The positron annihilation lifetime(PAL) spectra of implanted and non-implanted peanut samples were compared.