Abstract: H∞ control designs for T-S (Takagi-Sugeno) fuzzy systems have been studied, a new quadratic stability condition which is more simple and relaxed than Kim E. 's and a new observer design method for T-S fuzzy systems have been proposed. Then two new sufficient conditions in terms of linear matrix inequalities, which guarantee the existence of observer-based H∞ controller for the T-S fuzzy systems, have been proposed. The design methods are simple and consider the interactions among the fuzzy subsystems. Finally, we show by examples that observer-based H∞ controller designs for T-S fuzzy systems are very practical and efficient.