The effect of Sb on Ge epilayer grown on Si(001) is investigated in situ by RHEED and ex situ by X-ray diffraction.Without Sb,island formation takes place when Ge epilayer is 6nm thick,and the strain in Ge epilyer is totally relaxed.With Sb,the growth mode of Ge on Si is two- dimensional, and the strain is relaxed gradually.Even if the thickness of the Ge epilayer is 6nm, 42%- strain is still not relaxed.