In this paper, a necessary and sufficient condition for decentralized stabilizability for expanding construction of large scale systems is established which involves the computation of blocking zeros and testing a rational function for sign changes at these blocking zeros. Results for the scalar as also multivariable cases are presented and a systematic procedure for designing the stabilizing controller is also outlined. The proposed theory is applicable to a wider class of systems than those for which existing methods can be used. There are a few matrix identities established in this paper which are of independent interest in Control Theory.