Helium implantations are experimentally performed in order to observe the formation of vacancy defects in tungsten crystals. Characterizations with nuclear reaction analysis (NRA) and positron annihilation spectroscopy (DB-PAS) allow respectively to quantify the 3 He retention and to determine the nature of the vacancy defects formed during the process. Two types of defects are observed as a function of the impinging fluence: vacancies filled with He (HenV and HenVm>1) below 2.5x10 20 He.m-2 and small voids above. The influence of kinetic energy of the impinging He atoms and of the substrate temperature during the implantation is studied. Molecular dynamic simulations of the implantations are performed to further understand the formation mechanisms of the initial HenV defects. A good agreement is found between results obtained by simulation and experimentally. The implantation depth of the He atoms and the mobilities of He and W are identified as the main features governing the vacancy formation. 2