We have studied the vacancy type defects of the HgCdTe crystals by using positron annihilation time technique.Samples grown by Te solution method accommodate a large amount of Hg vacancies,no matter what kind of conducting type(n type or p type) they are.By suitable annealling process the as-grown p-type samples can be turned into n-type,and the traping of the positrons decreased(the positron annihilation time decreases by 14—17ps).If the samples are annealled at higher temperatrue,the positron annihilation time will increase.This indicates that the Hg vacancies are increased.The bulk time of the positron annihilation in HgCdTe we obtained in this experiment is 272ps.According to the positron annihilation time and the electric parameters of the samples,we obtain the appropriate annealling temperature is 180—220℃.