In this paper, the automorphism group of G is determined, where G is a 4 × 4 upper unitriangular matrix group over Z. Let K be the subgroup of AutG consisting of all elements of AutG which act trivially on G/G, G /ζG and ζG, then (i) InnG K AutG; (ii) AutG/K≌=G
1×D
8×Z
2, where G
1=(a,b,c|a
4=b
2=c
2=1, a
b=a
-1, [a,c]= [b,c]=1; (iii) K/InnG≌=Z×Z×Z.