The charge transfer collision cross section of He+ions in a collisional helium plasma has been determined based on the ion energy distribution at the target during the plasma immersion ion implantation (PIII) process. The ion energy distribution is obtained by inferring data from a secondary‐ion mass spectroscopy measurement and a simple collisional theory. The value of the charge transfer cross section in the collisional plasma was found to be higher than previously published values. The cross‐section value determined by the PIII technique was used in a plasma simulation and was found to agree with our experimental observations.