In this paper, the robust tracking performance for a class of uncertain plants with triangular structure is considered. A decentralized feedback configuration is adopted in order to preserve this triangular structure in the closed loop system and enable the asymptotic tracking with as less servomechanism as possible. The necessary and sufficient condition for the robust asymptotic tracking subject to this decentralized configuration is derived. Synthesis based on the obtained condition will yield servomechanism of order lower than that obtained by the Internal-Model Principle.