Using the descriptor standard form, the decoupling problem of the descriptor system by state feedback is studied. The descriptor system is first transformed in controller form and the necessary and sufficient conditions for the descriptor system to be integrator decoupled are obtained. Then, these conditions are shown to be necessary and sufficient for the closed-loop system poles to be assigned while decoupling. The characteristic polynomial of the closed-loop system is explicitly calculated and the conditions for the decoupled system to be internally stable are obtained.