The epitaxial growth crystal structure and magnetism of Co100-xMnx alloys grown on GaAs(001) substrate have been studied by using reflection high energy electron diffraction,X-ray diffraction and surface magneto-optic Kerr effect. For 0100-xMnx alloys.They show strong ferromagnetism.For 42100-xMnx alloy exists in bodycentered cubic(bcc)and face-centered cubic(fcc)phases respectively,and the coexistence of the bcc and fcc structures is observed in the middle stage.It is proved by using wedge samples that only the bcc phase contributes to the ferromagnetism.For x>78,the epitaxial alloys have fcc structure and show no Ferromagnetism.