Abstract: In this paper, the 100 keV helium ions were implanted in tungsten. The behaviour of helium and the evolution of the related defect have also been studied as a function of helium fluence by slow positron beam analysis (SPBA). The experimental results indicated that helium ions irradiation under different helium fluence introduced the same vacancy-type defects on the basis of the phenomenon that the S-W date present the same linear distribution. Moreover, with the increment of helium fluence, the concentration of vacancy-type defects in the samples increased correspondingly. It was also found that the S parameter of the sample annealed had a significant change by comparing with other samples without annealing. The result suggested that vacancy-type defects were more likely to be affected by thermal treatment than other influencing factors such as fluence.