Positron lifetime measurements, carried out over the temperature range of 10-300 K, have been used to investigatedefects in two undoped semi-insulating InP samples. The positron lifetime spectra were analysed by both PATFITand MELT techniques. The results at room temperature reveal a positron lifetime of around 273ps, which is associated with indium vacancies Vin or Vin-hydrogen complexes. The positron average lifetime is temperaturedependent and decreases with increasing temperature at the beginning (≤ 80 K and ≤ 120 K), and then remainsunchanged, which is attributed to the influence of negative vacancies and detrapping of the positron from thosenegative ions of Mg, Zn, Ag and Ca with ionization level (1-).