Cyclin B1(Ccnb1) is a key regulator of the mammalian cell cycle, which binds its catalytic partner cyclin-dependent kinase 1 (Cdk1) and play essential role in the G2/M transition.While its function during mitosis in somatic cells has been extensively studied, its function during both mitosis and meiosis in the male germ line is poorly understood.The embryonic lethality in conventional knockout Ccnb1 mice precludes determining its function in male germ line.Here, we use cre/loxP system conditional knock out Ccnb1 function specifically in the male germ line at different stage of male germ cell development to research the function of Ccnb1 in spermatogenesis.Deletion of Ccnb1 using Mvh-cre results in spermatogonia can not proliferate normally, germ cell lose before the first spermatogenesis cycle and male mice infertile.In contrast,deletion of Ccnb1 using Stra8-cre have no effect with spermatogenesis and male mice fertility.These results indicated that Ccnb1 is essential for mammalian male germ cell mitosis but may redundant in mammalian male germ cell meiosis.