The diffusion coefficient of As in MBE HgCdTe epilayers was studied by using secondary ion mass spectroscopy (SIMS). The diffusion was investigated at various temperatures of 240℃,380℃,and 440℃. It was found that the partial pressure of Hg could influence As diffusion and Hg vacancies in epilayers assisted fast diffusion of As. The As diffusion at a lower temperature of 240℃/24～48 hrs was negligibly slow. At a higher temperature of 440℃/30 min, the diffusion was obviously fast, which can cause the steep PN junction to become graded. By considering electrical activation of As acceptors and diffusion problem during annealing, an annealing condition of 440℃/30 min was found to be optimal.