  We show that the transformation of D-branes under T-duality on four-torus is
represented by Nahm transform of instantons. The argument for this allows us to
generalize Nahm transform to the case of orthogonal and symplectic gauge groups
as well as to instantons on Z_2 orbifold of four-torus. In addition, we
identify the isomorphism of K-theory groups that realizes the transformation of
D-brane charges under T-duality on torus of arbitrary dimensions. By the
isomorphism we are lead to identify the correct K-theory group that classifies
D-brane charges in Type II orientifold.
