Abstract: Given an output partition, finding (if possible) a feedback control law such that the compensated system is decomposed into several parellel and indepehdent subsystems this is the usual formulation of the block decoupling problem of control systems. For nonlinear input-output control systems described by differential algebraic terms, we show that they possess their own inherent decoupling structures. A system can be decoupled if and only if the given partition "matches" the decoupling structure. And, for nonlinear systems in state space equations, we can explicitly construct the decoupling structures by emploping the dynamic extension algorithm.