in this paper ,we give that a jordan block Jm(λ) has square -rooting matrices if and only if m=11 or λ≠0,Jm(λ) has exactly two square-rooting matrices::±A,where a1 = c , a2 = 1/2c , a3 = -1/8c3 ,… ,
ak = -(a2ak-1+a3ak-2+…+ak-2a3+ak-1a2)/2a1 ,… (k=3,4,…,m), c2 = λ.