  We show that the NCOS (noncommutative open string) theories on torus $T^p$
($p\leq 5$) are U-dual to matrix theory on torus with electric flux background.
Under U-duality, the number of D-branes and the number of units of electric
flux get interchanged. Furthermore under the same U-duality the decoupling
limit taken in the NCOS theory maps to the decoupling limit taken in the matrix
theory, thus ensure the U-duality between those two class of theories. We
consider the energy needed for Higgsing process and some bound states with
finite energy and find agreements in both theories.
