The vacancy formation energy E(v) of Zn and In are determined by using the positron age-momentum correlation (AMOC) technique. The S-parameter S(v) for the trapped positrons that is necessary for the analysis is obtained by extrapolating the time dependent S-parameter S(t). Thus it is not necessary to heat up the sample so high that all positrons arc trapped in vacancies. The E(v) of Zn thus obtained was in good agreement with the reported value. Also we obtained the E(v) of In, for which the previously reported values are scattered over a considerable range.