Let M i be a compact orientable 3-manifold, and A i a non-separating incompressible annulus on ∂ M i , i = 1, 2. Let h : A 1 → A 2 be a homeomorphism, and M = M 1 ∪ h M 2 the annulus sum of M 1 and M 2 along A 1 and A 2 . In the present paper, we show that if M i has a Heegaard splitting V i ∪ S i W i with distance d(S i ) 2g(M i ) + 3 for i = 1, 2, then g(M) = g(M 1 ) + g(M 2 ). Moreover, if g(F i ) 2, i = 1, 2, then the minimal Heegaard splitting of M is unique.