Abstract: Microstructure of homemade modified 316L stainless steels and its thermal annealing behavior are studied by the positron annihilation lifetime technique. There exist monovacancies, divacancies, dislocations and small vacancy clusters in them. The monovacancies, divacancies and dislocations are annealed away at 400, 600 and 800 ℃, respectively. The observed vacancy clusters are quadrivacancies and pentavacancies. The relative intensity of pentavacancies increases with the increasing of annealing temperature below 200 ℃, dissociation takes place and the relative intensity of pentavacancies decreases above 400 ℃, and up to 800 ℃ quadrivacancies still remain with a low concentration.