Exception is a kind of important rule in WTO which balances trade and non-trade objectives. It limits general rules or principles setting obligations. In WTO disputes, the respondent may invoke exceptions to defend the legitimacy of their measures. The WTO panels and Appellate Body established a number of standards to identify exceptions, such as differentiation between exceptions and exclude application rules, consideration of the nature and importance of the rules as well as the attitudes of the parties and so on, but in practice these standards are not fully complied with. Affirmative defence is a concept in close contact with exception in dispute settlement practice. In general, there are no essential differences between exception and affirmative defence, they are basically used in much the same way by the panels and Appellate Body. China should make ??more use of exception as defences in future disputes, which would greatly benefit China if supported by the Dispute Settlement Body.