We shall characterize the U-convexity of the psi-direct sum (X(1) circle minus...circle minus Xn)psi X(n))psi of Banach spaces X(1),...,X(n), where psi is a continuous convex function on Delta(n)(= {(t(1), t(2),..., t(n-1)) is an element of R(n-1) : t(j) >= 0 (for all j)(,) Sigma(n-1)(j=1) t(j) <= 1}) with some appropriate conditions.