It is confident that mammalian somatic cell and embryonic stem cell can form whole individuals if they are transferred to enucleated oocytes at M phase. However, a large proportion of clones were stillborn or died within several months due to various reasons. Morphological abnormalities have also been observed in clones. The exact reasons for such abnormalities are not clear, but genetic or epigenetic modification in donor cells and insufficient reprogramming of nuclei due to improper nuclear transfer technique are considered. In this review, we discuss the present status of animal cloning and reprogramming mechanism of nuclei.