Effect of activation annealing on composition of As-doped HgCdTe epilayers was studied,the experiment was done on different heat treatment conditions and on epilayer with different thickness,and the infrared transmission spectra measurement was used to compare the composition change of epilayer before and after the annealing process.It is shown that the thicker of the epilayer,the slighter influence of activation anneal on the epilayer composition on the same annealing condition,at the same annealing temperature,the composition increases gradually with the annealing time and smaller composition change was observed in thicker epilayer.